I'd remortgage my current house, which should release about £100k.
That gives me £200k in total, which I would then put down as a deposit towards a bigger house, I want 4 bedrooms as a minimum.
I would then put my current house on rent.
Hence I would have my new house, and have my current house as an investment - since it's a 3 bed house with garage, and a primary school, playground for kids, and bus stop into town literally 45 seconds walk from the house, I should get a small family as tenants pretty easily, who are more likely to settle for a longer period of time.
Well that's my goal for 5 years from now anyways, whether I have the spare £100k or not!