Trying to get the combination of things you’re after looks quite difficult from a short google. There’s an Octane frame on CRC for £199 but it’s 142 rather than we and needs a 120mm fork.
What fork is in the current bike – could it be extended to 120mm travel with a new airshaft?
Wheels might be more tricky as most of the mid range boardman’s don’t run big name wheels that you can easily change the axles on.
The Onza payoff frame looks like it’s available for £199 too – but again for a 120mm fork / bolt through rear hub. Also has a BB30 bottom bracket standard so might be even less compatible.
I think the German brands are most likely to have something compatible – maybe Cube / Felt? They come up as bargain frames from time to time with the various outlet online shops in Germany.