No evidence of reed or bowed instruments in Britain at that time, so far as I know. Doesn't necessarily mean there weren't any, but a lot of what you'll read out there is pure speculation (and sometimes pure fantasy!) Probably simple flutes, and small harps or lyres. Percussion of some kind. Maybe (and maybe not) some kind of horn.

As several people have already said, it's pretty much impossible to reconstruct anything remotely authentic for that period in Britain; we just don't have the information. There's no evidence, incidentally, that the music of the time will even remotely have resembled what is thought of as "Celtic" nowadays, nor that it would mean much to the modern ear.