http://www.internaxx.lu/
Very few brokers charge high-rates of commission anymore (As outlined earlier as 5GBP, when was the last time you traded? The 80s?). Regardless of how many shares you buy or sell or even short internaxx and most other dealers will charge a few pence, as long as you do it over the internet yourself. No stamp duty that way either.
200 GBP is not a lot to start out with however, and realistically you should start in around the 800 GBP range. Also, I would suggest you read up on it and do your own trading rather than trust some schmuck with your money. They see someone with little money to invest they'll dump one of their expenses into your pocket, whether it be rubbish stock or otc options.
Now's not a great time to get into the markets with all this volatility, small time investors usually get their knees broke in times like this, so I'd say just watch it for a while before you do anything. Channels like Bloomberg or CNBC will keep you sort of up to date with developments, so they're your best bet.