I’m looking for a new Ultegra Disc groupset for my adventure bike frame. My frame has a flat mount rear brake, and my fork has a IS brake mount.

Merlin only sell the flat mount group for £638. Evans only sell the post mount group for £710.

Additional consideration: I’d like whatever I buy to be compatable/adaptable to use on different frames in the future.

Which groupset should I buy?

Bonus question: my frame is for band-on front mech. The Evans group only has braze-on front mech. Is there any disadvantage to using a braze-on front mech with a band-on adapter? e.g is there any loss of stiffness which results in poorer shifing?

My head is spinning from all this, any guidance gratefully appreciated!

Buy the cheaper flat mount set-up, and buy another caliper for the rear. Keep the extra flat mount as a spare, or flog it on to fund the other one.

Post Mount brake calipers can be used on flat mount frames / forks.
Flat mount calipers can’t be used on Post Mount frames / forks.

e.g is there any loss of stiffness which results in poorer shifing?

Yes, a bit.