Parkers are local to everyone...but expensive...
ISG are good...
If you google steel stockholders you will get the nearest ones....
BUT...
The delivery charge is gonna be the killer...steel comes in 21 (ish) foot lengths normally...so big truck and delivery charge or a couple of lengths of box section will be as much as the steel nearly
So your best bet is gonna be to see if your local frag yard will let you buy stuff off em, have a rummage and buy at scrap price offcuts that engineering company's have weighed in that will do the jobby for you...costs on a couple of quid for stuff that would cost ten times more at a stockholder,
OR...
Approach a few local engineering companys...many will either add what you want to their next order, saving del charges, or might even sell you some of there steel...this means you can get it their wholesale price, or maybe even for a drink if it is what they consider offcuts from a job...
They might even cut it to length for you if you take your measurements with you...
