I just found a FAQ page on Ticketmaster with info about the UK vendors for the presale:
GLASGOW: Ticketmaster is the official partner for the SSE Hydro and will be running the presale for standing tickets, seated tickets, (RED) Zone and VIP packages at this venue.
LONDON: For The O2 in London, the venue will be running the presale for standing, seated and (RED) Zone Packages (visit U2.com for more information) but Ticketmaster will have a number of VIP packages available for London during the presale.
So if you want regular London tickets in the presale, it's being handled by the O2 website, NOT Ticketmaster.
It also included the following information:
There is a limit of two tickets across the tour per customer. (No word on whether you can split them between shows.)
Please note that for the floor standing tickets in Glasgow, Ticketmaster will be selling Paperless Tickets which require the cardholder to attend the event.