I think our specialization is an advantage because you have a higher level of understanding than our US counter-parts in a specific subject. Undergraduate degrees should really be preparing you for academia, they need to stay focused enough that a masters or PhD is a feasible follow-up. However I can see how they frustrate most students who aren't on that path.
Most degrees only require 1 or 2 specific subjects so your third a-level choice can provide you another option. I did Maths, Physics and French but to get into my degree I really only needed Maths, I just like Physics. I personally felt more restricted by my grades: by taking French I lost the option for a coveted AAA as French is my worst subject (I got a B). By taking French I restricted my university choices but I was happier. I still learn French as well so it's done me a lot more good than Chemistry ever would!
A joint degree is probably a good option for you: Maths and Philosophy for example. A quick google has revealed they only require Maths (sometimes FM) at plenty of universities. You may enjoy a Scottish uni that has an extra year and generally allows you to take 2 or 3 different subjects in first year (there are other unis in the UK that do this, I just know it's standard in Scottish ones).