Amazon went back to £34.99, but I think I might have a way to get it for approximately £20 through a trade offer although you would need about £60-70 initially since it's not a straightforward deal.
I can't post links, so bear with me on this.
Gamestop UK are doing the pre-order for £39.97, however trade in 2 games from their list (gamestop/gtavtrade) and they refund £35, leaving the total cost at £4.97.
Considering some of the titles on the list can be bought for cheap, this could mean buying a couple of games to take advantage of the trade offer and ending up spending a total of £20-25 once the refund is accounted for.
I've got my eye LBP Karting from eBay for £7.29 along with GTA 4 for £8.74
£39.97 for the initial order of GTAV along with £7.29 + £8.74 for the games I intend to trade comes to a total of £56. Once the refund of £35 has been processed, this means I will have spent £21 on getting GTAV. Considering the list has loads of stuff, it might be even cheaper for someone who has access to other games.
It's a fair amount of hoops to jump through, but that's a saving I'm not going to ignore.