I'm building a single/fixed gear bike using an old english frame i aquired. I guess the bottom bracket is either Raleigh (1 3/8 x 26TPi) or British ISO (1.37" x 24TPi). Is there an easy way of telling which one it is? The bottom bracket is 68mm wide if this helps??

You haven't told us what kind of frame it is...

Here you go, from Sheldon Brown of course.
"The width of the bottom bracket shell is also an indication. Standard British (24 tpi) bottom bracket shells are 68 mm wide, give or take a mm. Raleigh 26 tpi shells are generally 71 mm or 76 mm wide. "
https://sheldonbrown.com/raleigh26.html

I don't know, its a lugged oldish frame, prob English. It has a serial number but i dont know if its possible to find out with just that info.

I saw the Sheldon website too, so am i right in thinking that if it is 68mm it must be the Std British?

Not quite. If the shell is 68mm wide, it might be more likely to have standard 1.37 x 24tpi thread, but you can't be certain until you've threaded a known 1.37 x 24tpi cup into it.

There was a lot of variability in parts in the past. I have a number of British frames with 66mm, 68mm, and 70mm shells with 1-3/8" x 26tpi thread.

Brand and model aside, the only way to know for certain and to take all this guess work out of it is to take it to your LBS where they can use a thread gauge....

Or just grab a bb and a big spanner and go for it. The bb will either go in nicely or bugger up the thread thus making the question academic