Hi,
Some good points above!
The particular unit you have may or may not accept new maps, some of the earlier standalone units for instance couldn't accept for instance. You've not said which model you have so by looking at that, and cross checking with the TomTom website, you can determine if it can accept new maps or not. For you information, GPS signals are the same the world over as it's provided by the USA's satellites orbiting the earth.
Assuming it can, then you'll need to purchase the UK map and upload to the device. There will be instructions on the tomtom website to help you with this or, you can drop them a support email, in case you don't have the documentation that came with your device.
If it can't, then you have multiple choices as per above. Some GPS software solutions are indeed free, some paid for versions work much better than the free ones, other's have better interaction and usage and others provide better overall better value for money etc. Only you can decide which is the best solution for you.
I personally use Co-Pilot on my smart phone as I prefer to have less 'boxes' to lug around etc. Excellent value for money at £27 for UK mapping and £29 for USA and Canada maps. I tried some of the freeby GPS products and found them fiddly for entering data and speech recognition whilst good was sometimes quite innacurate, so a little opposite to Mariposa's findings.
Good luck whichever way you go!
Cheers, DtM! West London & Slough UK!