Try http://www.sketchup.com/ - it's incredibly simple to use and there's a community library of common furniture from IKEA et al. All for free!

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.)

Agree with Jonathan. Peninsula's always split the space in half. I think his layout is a much better use of space I think an L shaped sofa, unless the chaise/ottoman end is compact is quite a large piece for the available space. A two seater sofa and a chair would be more flexible. You could also simulate the L with two sofas with a side table between them. Think about how you live, what suits your lifestyle? If you had a rectangular table it could double up for kitchen prep running parallel to the hob.

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.