I'd go with Chuffy's advice.
Frankly there's an awful lot of crap talked about bikes as with many things in life.
All things being equal would a Titanium or Carbon framed bike with say Veloce and whatever wheels be any quicker/more comfortable than your 531 with the same kit...well marginally I would say, but so marginal for the average rider on the average UK road. OK the
Evans would weigh a bit more, and would be marginally less stiff (though read Sheldon/Brandt and there's and argument that says stiffness does not mean faster or more efficient). 531 frame and Fork combo's are generally comfortable too.
Spending a grand will give you a lovely bike just like all the other lovely carbon/Titanium stuff out there.
Spending considerably less on whatever upgrading route you choose (minimal a la Chuffy or new wheels, tyres and groupset ~£400 to ~500 tops) will givew you a bike that's hand-built, fast, comfortable, with a history and
unique.
A modern Cro-moly frame (not quite up to 531) will set you back at least £400 with look at Thorn's new Audax MK 3 or Salsa's Casserole (a C+ Best-Buy).
So, don't think the frame is not worth upgrading, just think about how to do it. Otherwise let me know what size and I'll bung you a few quid for it...I need a new Fixed project...
Depending on the quality an wear of the existing running-gear, a new groupset will generally be smoother and shift cleaner and more accurately, but for mere mortals won't really make you go much faster.
Weight issue. Weight is an issue when either accelerating or going uphill, once up to speed, weight can be a benefit. Unless you are criterium/club racing, don't get hung up on a slightly heavier frame...the place to save a few hundred grams weight where it really makes a difference is the wheels and you'll really notice an improvement in Sprightliness.