Agree the OFCOM site is lousy. If you want to get a number on-line, you first need to register on the site. OFCOM will then send a password, by letter post, 5 days. This allows acess to your boat radio licence records. In my case I then deleted reference to my non DSC VHF and ticked the box for a DSC radio. The MMSI number appeared instantly. You can also print off a copy of your new licence.
I believe that you can also apply by 'phone.
I cant recall having to do any of that. Even my old annual ships radio 'disc' had an MMSI number on the paperwork. I renewed by post in July(?) and my lifetime licence came through, with the MMSI number.
I too was lucky enough to get a snailmail lifetime licence. From the vituperation heaped upon the Ofcom site, seems I was saved some wailing and gnashing.