Hi Will

As you saw, the best place to start is the Build Diaries and reading through the posts in the corresponding weight class. There is no point in building a Lightweight in the UK though as no one has an arena to run them. The most common starter classes in the UK are Featherweight, Beetleweight and Antweight, and once you get there you can build a Heavyweight.

Springs have been used in the past but not recently. They will pack less of a punch than pneumatic weapons but I can see them still having enough power to flip opponents or even toss them out the arena. It would be great to see a spring based flipper! You would also have the advantage of, pretty much, unlimited flips, where as a pneumatic robot probably has a max of ~25 flips in a fight (That is a total guess, but its limited regardless)

There are teams based in Leeds, Sheffield and Birmingham so you are not the only person in that area. I'm a little further north near York. I am sure some one on here will be close enough to give you a hand if you need it.

Good Luck!