Cheapest ever by a fair margin:http://uk.camelcamelcamel.com/Crucial-CT1024M550SSD1-Internal-Solid-State/product/B00IRRDHW6
The M550 is Crucial's latest model that comes in 1tb. This is very likely the best 1tb option on the market.
Currently, Crucial's last-gen and slower 960GB M500 is the same price, which makes the M550 the no-brainer. Another option at 1tb is the newly released Samsung 850 Pro. It's faster and more durable than the M550, but it's supposed to be priced at between £500 and £700 - double the price.
The Samsung Evo 1tb is the only real contender. It uses a more cost-effective, but much less durable technology called TLC NAND - it should be cheaper than the M550, but it's currently the same price. The 840 Evo is quicker than the M550 at capacities lower than 256gb, but not at 1tb. They trade blows in benchmarks; they're the same speed. In addition, the M550 has built-in data protection on power loss. The M550 is undoubtedly the better option.
It's in stock in 3 days, and sold directly by Amazon with a 3 year warranty.
Edit: Now reduced to £299.99