This is a question I am often asked and there is no simple answer.
A hardy palm tree that sees a brief low of -10c with a daytime high above zero is far less likely to be damaged than one which spends a week or more in day and night sub zero temperatures with lows of just -7C or -8C.
Below I will try to give explanations of the various hardy palm species limits and when to protect.
Trachycarpus fortunei and Trachycarpus wagnerianus are by far the most suited palms to the UK climate. They can be planted in the ground and forgotten about throughout most of the UK and generally require no protection whatsoever.
There are many mature 100+ year old specimens throughout the UK as living proof of their suitability.
The only occasion in my lifetime when I saw plants damaged or killed in the UK was in the winter of 2010 when temperatures reached -18c in parts of the country, but most survived.
Trachycarpus species other than the above remain relatively untested , Trachcyarpus nanus seems bulletproof, Trachycarpus princeps seems as hardy to me as fortunei and wagnerianus. The remaining group of takil, ukhrulensis ( manipur) and oreophilus are probably less hardy , and seem to take leaf damage at around -11C, although thats still plenty hardy enough for most of our winters here in the UK.
Chamaerops humilis , vulcano and cerifera. These palms seem hardy to around -8C although cerifera seems a little hardier than the others. Even so, losing a spear in winter is not necessarily the end ,they will often grow out again from the affected trunks. These palms do better in sunny well drained spots or under walls. It is also the best palm for a pot , which of course can easily be moved under the shelter of a house wall in winter. I tended to throw blankets or tarpaulins over them should a night of -8C be forecast.
Rhapidophylum hystrix, reputedly the worlds hardiest palm , and does very well in shady damp spots in the garden where it will slowly produce lush glossy palm leaves more akin to the heat of the tropics. This is one for most of the UK if you have the patience to watch it grow slowly.
Brahea armata and Brahea edulis. These are 2 very underated palms for the UK. Certainly hardy to -8c possibly a bit more.
They will require a nice sunny spot with excellent drainage but will require some protection during prolonged severe cold spells where day and night temperatures do not rise above freezing.
Washingtonias show some hardiness , certainly there are impressive specimens in mildest parts of the country. The limit is probably -5C before showing damage, although quick growth aids recovery. Difficult to protect as this palm hates trapped moisture and quickly succumbs to fungal infection. Unless you live in a milder coastal area or inner city heat island, its probably one to leave aside.
Jubaea chilensis is the hardiest of the feather palms by far , as well as being the worlds biggest palm tree. There are records of it surviving -18C in France although I suspect this was a short lived freeze as trunk damage is sustained well above this temperature. Certainly -11C or -12C is not unreasonable to expect as a limit, and they are capable of recovering from lower temperatures even if the leaves are burnt. This palm can be planted in a sunny well drained spot , given space to grow and only needs a little help in the most severe of winter cold spells in the UK. Certainly , unless the temperature was expected to pass -10C , or a severe and prolonged cold spell of day and night sub zero was forecast I would leave it to its own devices.
Butia odorata ( sold generally as capitata) , Butia catarinensis and Butia eriospatha. All of these Butias are leaf hardy to around -8C to -10C , eriospatha is reliably a degree or two more cold tolerant. They should be planted in sunny well drained spots and benefit from raised beds. Butias are not root hardy , so in pots require robust pot insulation, a hard frozen pot usually means a plant that dies in spring. Butias can be left to fend for themselves throughout most of england and wales during winter, but should be protected during those rare weeks of prolonged day and night sub zero cold.
Butia yatay is altogether another beast, something of an enigma, on paper it should be hardy as any other Butia , but it really seems to struggle in our climate, I do wonder if the heat requirement is too great for it to grow healthily and sustainably.
Phoenix canariensis is hardy to -8C at which point it burns and quickly dies. Spear pull usually results which is fatal.
Its a very fast growing palm so can easily recover from and discolouration to leaves in winter. However its huge size and spiny nature makes it very hard to protect and its not at all suited for small gardens. Its a truly magnificent palm for mild coastal locations or inner city locations where it has space to grow and where temperature remains above -8C. I would recommend avoiding it unless you live in a milder part of the country , as the overwintering and protecting of it is not at all easy.
Protecting a palm also requires some personal judgement, given the above factors. A freeze in late november is far more serious than one in late march with spring about to start. Damage sustained in November is likely to kill a palm if fungus progresses throughout winter.Warm spring sunshine shortens the frost duration, dries the plant and seems to enhance hardiness. Moisture levels are important, a sopping wet palm that freezes suddenly is likely to sustain damage in the centre growing spear. When deciding whether to protect a palm tree all these factors must be considered, as they can affect the palms hardiness levels.
Hopefully this summary will help people with hardiness issues, I cant pretend its a definitive guide and some people will undoubtedly disagree with me. Protection methods will be dealt with in another article.