Wheels will make the most difference. Stan’s Flow on Hope will save some weight, or try Superstar hubs for a cheaper option.
Chain set – SLX gives by far the best balance of weight, performance and price.
Decent 160mm forks, which sounds like what you need, aren’t cheap. I just bought some X-Fusion Vengeance which are prob the cheapest of the new options, and they’re very good so far. Otherwise, look around for s/h Lyriks or Fox 36s but be careful what you buy and be prepared to add the cost of a service to the price at a minimum.
As far as the shock goes, there’s a bunch of options. I’m no expert but have always had good results from Fox air shocks. The Cane Creek Double Barrel air gets good reviews but it’s pricey.
Other than that, think about buying a whole bike either second hand or a discounted last year’s model. I think Rutland Cycles are knocking out a Lapierre Spicy for about £1200 at the mo, which might be worth a look. Something like that may end up being your best and cheapest option.