Are you sure it's not flat because of a leak? Are you sure that the first valve is aligned correctly? A "full tone" flat seems WAY too much to have escaped the factory as part of the construction of the horn. By "full tone" do you mean a half step (50 cents)? I would be totally shocked if the cause of this was a 1st valve tuning slide that was too long.
Don't cut the horn unless you KNOW what the problem is.
And if you're anywhere in the continental US, there are several excellent repair facilities to ship (or drive) it to that are much better options than shipping it to England.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)