Lizzy.
Don't make the bathroom any smaller.
As for kitchen. Are the carcasses (insides) of current kitchen ok?
If yes then just change the doors and get new worktops fitted. It will look like a new kitchen but for a fraction of the cost.
If the current kitchen isn't ok or the layout is completely wrong then I would look at IKEA. Esp if you will be extending in a few years. You will get a lot of kitchen for your money.
Given that you're kitchen isn't huge you won't have to spend a massive amount.
Bathroom is fairly small too. The upside is that it saves on costs.
Do you want a shower separately? You could just squeeze it in. However it adds to cost.
Have a look at Bath Store. Also look at eBay as lots of plumbing suppliers sell there and are cheaper than shops for the same thing.
I think your £15k will easily do it.
Also if your bathroom as a really good extractor fan and a window, then don't waste money fully tiling it floor to ceiling. Just tile around sink, bath and shower.
This also allows you to change the colour of the walls in a couple of years time if you fancy it.