One of the big themes in Peace Talks is misdirection or misunderstandings.

Harry makes the assumption that Thomas's condition is because the Svartalves beat him up. What if that's not actually the case? What if Thomas was beaten prior to the attack?

Now this sounds like an odd thought, but we have confirmation from River Shoulders that the Genoskwa Blood on His Soul is still around, and gunning for Harry. In the last book, Skin Game, we are shown a being capable of mimicking another down to almost the smallest detail. Vadderung mentions that there are 3 other beings capable of that in the world - one of which is presumably the Naagloshii from Turn Coat. We've got many call backs to prior novels, and this would be yet another one.

So my tinfoil theory is that Thomas was taken and beaten right after he left Harry's apartment. Harry leaves to visit Murphy. A shapeshifter takes Thomas's form, with the Genoskwa carrying in the beaten Thomas under a veil. It makes the (deliberately failed) attack, and drops the beaten Thomas in one of the many Svartalf traps as the patsy, and escapes in the chaos with the Genoskwa under a world-class veil.

I personally agree that Arthur is probably the likeliest candidate, with Merlin being disavowed by WoJ.

Presumably, Arthur would have a bit of a language barrier, but then we have the whole 'Piss Off' comment, like a guy trying out a slang term he's unfamiliar with. With Merlin apparently running through time all over the place, picking up a language and filling in Arthur on it doesn't seem too convoluted. Particularly if Merlin had some long-term plan in motion.

Another thing I noticed is right after Harry interacts with the British Figure, he suddenly gets a Parasite Headache, that takes an unusually long period of time to be resolved by Demonreach. The paranoid side of me wonders if that may have been deliberate on the part of "Alfred", to throw Harry off of too much investigation into an area that might have endangered Merlin's planning.