Carbon if over 1K, Alu if below, not too much in steel now, unless you pay lots for the really good stuff.
What ever you look at, if possible would try to get one which has a rear hub can take an 11 speed cassette for future proofing.
From where you live would look at a standard crank (53/39) you won’t need a compact in East Anglia but most sub 1K bikes seem to be spec’d with compacts. And avoid triples, too much trouble for so little gain in gears, and not needed in East Anglia.
For 2nd hand vs new, there are some good 2nd hand bikes out there, but you will have to search, as you are close, would be worth looking at Pauls Cycles, as they have a lot of closeout models at deep discounts