STAR is the The Society of Ticket Agents and Retailers
SeatChoicehas links to the STAR website and the STAR Code of Conduct.
However, even STAR Members are allowed to add on sometimes quite huge fees.
(I gather there is an upper limit, expressed as a percentage of the ticket's face value - though I can't remember what percentage.)
That's why I'm so impressed by SeatChoice:
it lists each supplier's exact fees.
Also for each search it shows exactly which Agencies are included in the results -
ie we know what all those included would charge,
so we only need to look independently at any other sites we might know.
(And No, I'm not in any way connected with the site: I'd never heard of it till today.)
-------------------------------------------------------------------------------------------------
'London Theatre Direct' seem to be reputable - they're certainly STAR members.
I think I've booked with them myself (by phone) -
& if it was them I'm thinking of, I was particularly impressed with the very friendly and helpful response.
'UK Theatre Web' are also STAR members -
and I think (I'm not sure) might provide some or all of the WhatsOnStage Tickets &/or Ticket Offers?
They're connected with Encore, who service (if that's the right word) many of the ticket offer sites -
I'm (almost) sure I've booked with sites connected with Encore - probably many times!
(I certainly look at their various offers almost every time I'm booking!)
'West End Theatre Bookings' I've perhaps not used. (I was mixing them up with another company with a similar name.) On full-price tickets their fees look hefty - though on the plus side they're Founder Members of STAR.
They look to have good Offer prices, well presented - although like lastminute.com the (random) offers I've clicked say Best Available, ie don't specify Stalls or Circle. There's a 'Call Me Back' telephone option though - and (though not at this time of night) live help chat.
I don't think I've come across "London Tickets": maybe they're new? I see they have the STAR logo.
They too seem to add hefty fees on their full-price tickets.
They do have some offers pictured, though they don't seem to specify what dates the offers are for -
& the few random dates I tried didn't produce any offer prices.
NB The above are my thoughts & experiences, as requested: I accept no legal liability for any misconceptions etc!