Not sure if its mainstream music or specialist that you want authenticated so difficult to give correct opinion but I would defin start off with Frazers Autog raphs, the Strand. One of the chaps there , in my opin, is v good & honest & thorough. It is expensive to get their letter of authentication , altho cheaper if it is a bunch of items by the same band/singer but in any event if you book an appoint & take stuff in I 'm sure someone will give you quick opinion re sigs . The other good thing about Frazers is they are used to e bay sellers fraudently using their name to authenticate fraudulent stuff so nothing fazes them
Bonhams branch in Knightsbridge do specialist music sales & they have just had one with mainstream and non mainstream bands/groups - however everyone is that department is very unfriendly & not v helpful
However, I'd try Frazers first altho I haven't a clue what kind of music you're in to
Pl bear mind this is what I'd advise if we were friends in real life & you were asking for advice but obviously I'm not responsible for how this turns out . However, Frazers have always been v pleasant & helpful to me & do have a very good reputation for authentication & do do their research . One of the guys wasn't sure about a sig. I left it with him & he consulted & did research before making his decision re it.
Whoever you choose it is better to take the stuff in , so they can see it face to face, rather than send a scan because they can tell all sorts of things by actually looking at the sigs whether they are familiar with the group/singer etc