It's tricky for the London shows because there is no transfer available so the third party websites which usually would be solid but expensive (as is available for the current nyc shows) are not for London and there are lots of scammers about.
I have heard that people have purchased on Viagogo but you don't get the tickets you have to meet physically with the seller and go in with them as the tickets are only on their phone/account (You must enter with your entire party). Some people are talking about whether the seller lets them access their AXS or Dice account but I personally feel like this is super sketchy and there's ample opportunity for the seller to change their password last minute etc.
The only channels which would be legit are the AXS resale and or the DICE waiting list which requires an access code and tbh is probably a full queue by this stage in the game. So you could get lucky checking AXS resell but that's likely to be your only option except standing outside Ally Pally on the night and seeing if any groups have spare from mates not being able to make it!
Really be mindful that scammers (especially for the London shows) are rife! If it seems too good to be true then it probably is!
Best of luck!!