In short:
UK cars are all TT
SZ - imported NA
RZ/RZ-S - imported TT
All imported cars will get roughly the same insurance quote - none of my quotes were cheaper when I was comparing NAs to TTs. UK cars will be cheaper to insure. All will be roughly the same to run.
As has been said, UK cars are rare (especially manuals) and therefore more expensive. Expect to pay roughly 3.5-4k for a good NA, 6k for an imported TT and 8k for a UK.
Obviously there are variations based on condition, age and gearbox - manuals are more expensive to buy although they cost the same to insure.
At 21 expect to pay around 1.3k-1.7k for insurance. Mods will add to the cost. I paid 1.2k on my imported TT for my first year, at 23 with 2 years ncb. My car had standard engine, but had full bodykit, aftermarket wheels, other exterior changes and interior changes, all declared.
Hope that helps.