Whilst there are precious few really hardy Citrus plants in the UK, some can possibly withstand a degree or two below freezing and given a sheltered microclimate for their home, might survive outside.
We can give no guarantees of hardiness and have seen some wildly optimistic claims about how low some varieties can survive.
We have had some survive quite a few degrees below freezing only to be completely killed by a two week north-easterly wind at one or two degrees above freezing.
Protection from cold winds is a major part of the key to possible success.
Wrapping with fleece should only be done when the temperatures are heading below freezing and should be removed the following day if possible otherwise the plant will not harden up to the cold and will be more susceptible to damage.
We have a number of interesting more cold hardy citrus for those wishing to experiment, but do think about when the last time you saw a lemon or orange grove in the UK!