"Personally I pay it direct to revenue myself."
"Surely I am entitled to keep that money in MY account till it falls due."
Only if you don't have a mortgage. If you have a mortgage, the solicitor has agreed with your lender that they will not release the loan cheque until the purchaser/borrower has put them in funds ( including stamp duty) to complete registration. Most solicitors have zero interest ( pardon the pun) in hanging onto client's money and, in the interests of speed and efficiency, generally pay the duty and lodge title for registration a.s.a.p to get the file completed.
If you don't have a mortgage, you can hold your own stamp duty but it still has to be paid.
mf