You don't need a new distribution board (consumer unit). You can either upgrade the wiring to 4mm and change the fuse (and fuse holder) to 30 A, or for a more simple solution, wire the circuit as a ring main, and fit a 30 A fuse. This is the best solution, find the last socket on the circuit and wire a 2.5mm cable from this back to the consumer unit and connect to the existing cable there. I'm assuming the circuit as it now stands is wired as a serial circuit using 2.5mm cable. Depending on the load/number of sockets on the circuit, you could probably get away with changing the fuse to 20 A, but be cautious of this approach.