Which steel road frames / steel bikes?

That's probably not enough. Custom built frames start at about £600 for a basic 631 (or equivalent). By the time you've added various touring bits to your specifications, you're looking at £800-1000 for the frame alone. Frame weight isn't the main consideration for touring.

Custom frame builders will not only take your measurements, they will also have a discussion with you about how you will use the bike and advise on components. Take their advice.

I suspect that small single person operations e.g. Dave Yates will not be that keen to build the bike up for you but would if pushed (paid). The larger businesses such as Bob Jackson or Mercian who also have shops would be quite happy to do it for you, at a price.

Agree about decent wheels. Consider discs.

Ultegra isn't really a groupset designed for touring. Many touring bikes use mountain bike components to get low enough gears.

PS Longstaff is dead (unless someone else has bought the brand and is building frames using the name)

Thanks for the heads up - appreciate touring bikes are usually mountain bike components. I'm keen to stick with road cycling components as I'd like to use this bike as a commuter as well - hence why i'm also concerned about weight. This is really going to be a bike for life so want to just buy one bike and stick with it - just want to have it quality and fit perfectly.