Good choice for a first gun. You might have already seen how much we recommend them. But yeah G&Gs are great for the money.
As Baz said, loads of places that sell them so take your pick really. Zero One would probably be my go-to recommendation but I wouldn't use their two-tone service (It's free, but shite apparently, which would explain why)
On the other hand you do get a free skirmish voucher when you buy from them which I think is great - it's like £20-£25 off that you're saving.
Wouldn't say you really need a pistol straight away, only just bought one myself after technically being an airsofter for like...3 years (I say technically because I haven't been to a proper woodland skirmish in a really long time)
Think about which pistols you like and try to get a feel for em (e.g. ask to handle one at next skirmish or local airsoft store)
Then make sure you're happy with the manufacturer. For example WE 1911s / Hi-Capas are the worst you can buy, The XDM is known to have lots of problems etc (Gen 4 Glocks are good) Whereas on the other hand. the TM versions are awesome.