Its true you could get around 280bhp but you'll need a lot more than that kit. I know someone that has recently used this kit in a build, they've already hit over 260bhp on the dyno and they are not finished yet.
You'll also need forged pistons and forged rods, may as well factor in new crankshaft and main bearings if you are going full tilt on a build like this so you'd also need a new engine gasket kit, cambelt kit, aux belt kit, advisable to put a new water pump in too.
The cat cam follower kit will require the head to be machined so all in all your looking at the thick end of £10,000
For that money you could have a meg conversion with over 300bhp, ohlins coilovers, a big brake kit, full ultra racing kit and a bit of change