Not unreasonable at all.
Gap do maternity lounging about things I think - so a big Gap might do maternity? They certainly sell babies and kids stuff.
You would think that big chain stores like H of Fraser, JL, M and S, Debenhams etc would find some space, given the B boom.
Here is a suggestion - if only on-line available, do a big batch of Click and Collect. Then take them into the changing rooms and try them on. If no good, return them immediately. Usually you can do this the day before and stuff will be in store next day. Money will go off cards of course, for a short period.