If you are borrowing to buy, then your solicitor undertakes to your lender that they will stamp and register the title and the mortgage. To give that undertaking, the solicitor must be in funds. I insist on having fees, outlays, and stamp duty in place on or before the closing date. I only once saw it happen - a solicitor I worked for did not collect stamp duty in advance and was left stranded by a client. Some clients can be very nasty.
If you are not borrowing, then the solicitor has no undertaking to give and you can decide not to pay until 28/30 days after the completion or never if you so wish. The problems with this are (a) solicitor will want to complete the file and not be chasing you (b) the title cannot be registered until the Deed is stamped and (c) if you are late, you will incur penalties and interest.
mf