Rules regarding Interest Only mortgages have changed significantly in recent months and they are now far harder to come by.
Most lenders will normally only allow a switch to interest only if you can demonstrate financial hardship exists and as this is a planned event I don't think the lenders will be very helpful towards you or your situation as they will feel that you should have prepared better for the maternity....yes I know, full blown recession, rising inflation reduced employment and working hours and you didn't manage to find that extra £20,000 of savings....how silly of you.....but this is what the lenders will assume you need for a pregnancy or that you will have full pay for a great duration of the maternity and not just statutory maternity pay.
It would always be worth speaking to the lenders to see what the lenders own position will be as it will change from lender to lender. The one thing that is almost certain is that they would limit any particular deal to a maximum of 75% of your property value and potentially 50% if it is for hardship reasons, while the first level is not a worry the second level would encompass your borrowing.
Its a hard thing to say but if your lender will not accommodate you then your best option may well be to grin and bear it. If you are able to move to interest only it would make a big difference as it will likely drop your payments from around £400 to around £100.
**EDIT** Instead of maybe looking to switch to interest only you may even be better off seeing if you can arrange a 3 month payment holiday with your lender. This would keep you on your repayment plan but would allow you to completely miss 3 months payments. the interest is rolled up and added to the mortgage balance but this would only be about £300 over the 3 months. tghis would not affect your credit score in any way and would be easy enough to arrange.....Not all lenders offer this service so it would be best to speak to them and see what they can offer.