Re: Question?
In practice, the three weeks tends to be only a guideline. In that period, the vendor's solicitor has to send contracts to your solicitor, your solicitor has to examine the contracts and ask any queries s/he thinks necessary and only then are you ready to sign the contracts and pay over the balance of the deposit. If you go over three weeks, especially in today's housing market, I'd say it's unlikely that much will happen.
Anyway, if your mortgage application has been correctly packaged with all the relevant information, it should take less than three weeks to get mortgage approval.
P.S. - you might get a better response level if you edit the title of your post to give a better indication of what your query is about.