There are more and more lenders that will now accept 10% deposits on new builds but there are still as many that will want at least 15% deposit.
Yor best bet is to go and speak to a mortgage adviser regarding this and see wat they can tell you about who to approach etc. This will also give you a view of the whole mortgage market insead of just the lender whose door you walk through.
Do remember that with a 10% deposit you are not going to get rates as keen as with a higher deposit and the underwriting will be a little tighter too.
I would always recommend that if you can manage a higher deposit then you will benefit from it but I do also understand that just because you need another £20k for a decent deposit does not mean it can be magicked out of thin air.
Good Luck