netfetter,
To elaborate a touch on the point Ross made, once the UK clocks went forward (last weekend in March) the UK moves one hour ahead of GMT, continental Europe two hours ahead.
The UK falls back into line with GMT for five months, usually during the last weekend in October - pretty much around the time of the Japanese Grand Prix every year.
http://wwp.greenwichmeantime.co.uk/
Is it confusing? It can be, if you aren't conscious of it.
So why GMT? Because it is the world time standard, it's as simple as that, honestly.
Hope this helps.