It's difficult - few lugged frames were made under 21" by British builders. If you can forego (or at least postpone) owning classic British steel then keep your eyes open on ebay for 80's/90's Italian frames* - they're little buggers and loads come up in the size range you're looking at. I'm keeping my eye out for my other half at the moment, she's 5' 3" but I'm fairly certain that she can get away with a 21" as long as it's a tad shorter in the top tube as she had a bike fitting session with Paul Hewitt last year.
*Edit to add - they should have forward facing horizontal dropouts, don't be put off if they look shorter than on older bikes as there's still enough adjustment to get chain tension sorted.
It's difficult - few lugged frames were made under 21" by British builders. If you can forego (or at least postpone) owning classic British steel then keep your eyes open on ebay for 80's/90's Italian frames* - they're little buggers and loads come up in the size range you're looking at. I'm keeping my eye out for my other half at the moment, she's 5' 3" but I'm fairly certain that she can get away with a 21" as long as it's a tad shorter in the top tube as she had a bike fitting session with Paul Hewitt last year.
*Edit to add - they should have forward facing horizontal dropouts, don't be put off if they look shorter than on older bikes as there's still enough adjustment to get chain tension sorted.