Could you put it under the stairs and knock through to use the pantry for size? or is it very low?

I'd also be inclined to swap them as Jonathan suggested. Check fire regulations, as perhaps you do need to rotate the stairs to allow an open kitchen. (I'm in the Netherlands so can't advise on these.)

We did something similar when we bought our forever home—also needed rewiring and plumbing, so I get the pressure on budget. Sometimes removing internal walls (especially non-load-bearing) can be more cost-effective than a full extension. You might want to check out home remodeling services in renton https://honeycombconstruction.us/ —they do thoughtful remodels and can help you plan it in stages to stay within your £300K.

Thanks for the floorplan! I don't mind keeping the kitchen as it is. But I do like the bathroom, bedroom layout! And definitely brining that wall out to create the 2nd bedroom which would then include the window.