As others have said, give serious consideration to not buying one!
However, if you must...
Tottenham Court Road would be your best bet. It's almost half way between Paddington and St Pancras, and if you don't have too much luggage it's probably a 30-40 minute walk from either. Otherwise, plenty of buses there, or you can take the tube (from Paddington walk to Lancaster Gate for a direct tube)
Tottenham Court Road has loads of computer and electronics shops along it. If you want somewhere where you can see the various options, compare between shops, maybe haggle, or otherwise get a good deal on an old model they're trying to shift, then that'll be your best option. Just make sure you're clear on what you want, and don't go for the first shop, plan a bit of time to check out the options, and have a good idea of approximate prices first!
Otherwise, you could look at ordering one online to be delivered to your hotel, either new or second hand. Oh, and consider the age of the maps - depending on where you're going a map that is a few years old might be fine, or might be a big problem. As an example, the Scottish Highlands haven't changed in a long time, but the area around Stratford and the Olympic Park has altered a lot recently!