Hi
Would agree about try before you buy, there are a few shops that offer this but can't think of one in London.
Not sure where in London your from but may be worth a visit to Lea Valley (www.lvagc.com) large friendly safe airgun club near Hertford. First Visit Free! All visitors get a 'tour' and a safety briefing.
On any given Saturday there are c. 50 members at the club so probably upto 100 guns around. Most members more than happy to discuss their guns. good / bad points etc and let someone have a few shots to get an idea .
We also have club guns available for daily hire of £5 which includes air & pellets (refills & air £1)
If you think you're going to come along give me a call to check it's not a competition day 07910 353111
Hope to see you soon
Roger Barrow
Treasurer
LVAGC