The combination of top speed and budget is going to narrow your choices a lot. You're probably looking at twin engines if you really want that speed. If you can drop your speed requirement a bit then there are rather more choices, alternatively you may need to up your budget quite a lot!
If you could live with a 40 knot top speed then an 8.5 metre Scorpion with a 300 Yanmar should do the job, or a similar size Ribtec if you can find one, or a Ribcraft. I think a similarly specced Redbay would probably do that sort of speed too. If you want to keep clear of the Yanmar then you might get away with a 240hp Yamaha in a 7.5 metre boat (I had the 165 in a 7.5m Ribtec and its max speed was 35kt).
Here's one that might just do the job though (its a few knots off, but well within budget): 750 Panthercat/Parker RS - Yanmar 300hp diesel - £28,000