Yep.
I've spoken to 2 mortgage brokers, entirely unrelated. And apparently (I found out 6 months ago) this is normal.
You take out the max cash possible from your current home. That pays off the remaining mortgage and leaves you with a nice lump left over.
The lump becomes your deposit for the next place you want to buy.
This is a Buy To Let (BTL) if you stay in your current place and rent out the new one, or a Let To Buy (LTB) if you rant out the old place and move into the new one.
Normally the mortgage on the place you're renting out becomes interest only, and the place you live in you make interest+repayment. This is most efficient re tax etc. It makes sense to plough all your money into paying off the repayment mortgage ASAP, as the interest payment is tax deductible. Although that is changing over the next 4 years, but even after that it will still effectively be a little bit tax deductible, so putting an extra £100pm into your repayment mortgage and leaving the rental mortgage as interest only makes most sense until you have fully paid off the residential mortgage.
I have now heard from the solicitor and apparently out issue is that our exchange-completion period is 2 months. 'Normally' it would be 2 weeks and they'd be happy to give the money early, but 2 months is a bit long.
Thanksfully we do actually have 5% in savings and have argued the deposit down to 5% (because of the long exchange period, sinking 65k for 2 months felt wrong) so we can pay in exchange and then hope the mortgage company don't change their minds before completion. If they do then we'll cross that bridge then...
There is no reason at all to think they will change our minds, the sums add up comfortably. However our solicitor advised us this company sometimes do.
All so stressful 