Any question answered...

You can shave weight pretty much anywhere on your current build, but not many of the things you can do will be good value in weightweenie terms. SRAM Red isn't actually worth the extra over Force unless you've already tackled all the big weight loss items, i.e. frame and wheels. If you want to make it lighter/faster for sunny Sundays without compromising its daily driver qualities, get some 'best' wheels, as you can quickly swap them in for nice days, and you can use nice tyres on your best wheels without worrying about £30 tyres that only last 1000 miles on your commute.

This is all good advice, I looked at Red and Force, and decided that Red was not worth ~£500 more than Force.

That £500 can get some pretty nice wheels, or bars/stem/seatpost.

The frame may be relatively heavy, but you can certainly lose a lot of weight from all the other components, and you can of course swap the frame at some point.

Honestly though- nothing makes a difference that is more noticeable than light wheels (in my experience, anyway).

Less inertia when accelerating really gives you a kick out of corners, up hills and so on.

But it is a good idea to have some training wheels for the day to day, and if you only have the space for one set then something like the EA90 SLX are a good compromise- light, no weight limit, clinchers.

When they come up on eBay they go for around £300.