There must be some alternative to "modest". Why are they not making it a sub brand and really embracing that market segment?
Ultra conservative would do it.
Ultra:
adjective
1.
going beyond what is usual or ordinary; excessive; extreme.
conservative:
conservative
kənˈsəːvətɪv/Submit
adjective
1.
averse to change or innovation and holding traditional values.
"they were very conservative in their outlook"
synonyms: traditionalist, traditional, conventional, orthodox, stable, old-fashioned, dyed-in-the-wool, unchanging, hidebound; More
2.
In the context of the UK and British women as a whole, (and we are in the UK, so I think that's relevant) these clothing requirements and restrictions can certainly be described as 'going beyond what is usual or ordinary, and in many cases are considered excessive or extreme.
And averse to change, hidebound, orthodox, traditionalist, unchanging? Yes to all of that. The fashions might change a little, but only within strict parameters that never change. That's the difference between so-called modest fashion and all other fashion.
So 'ultra conservative' does it for me. Or even just conservative, if the 'ultra' bit offends thee.
Not so nice when the negative value judgement is on the other foot though, is it?