Cockpit improvement

I don't think this is true - are you 100%? Pretty much every modern carbon fork will take a 25c and a lot would take a very tightly squeezed 28c (which I wouldn't suggest tho). From the ones I've seen on my commute they'd also take a 28c rear.

My vote would be for 25c tyres, checking your bike is set up correctly (including installing two proper drop brake hood leavers) then possibly carbon bars, and if you want to drop weight a lighter alu stem with ti bolts. When i was looking Deda and Ritchy were the best option for weight and price.

The brake bridge on the back of a Langster frame is very tight indeed- I couldn't get a 28 in there, 25 was the biggest I could go.