"The Celtic languages" - as Casey says, this is a linguistic group with common ancestry, not a single language. There hasn't been a Celtic language per se for a long, long time :-)
In no particular order:
- Welsh
- Scots Gaelic
- Cornish
- Manx
- Irish
- Breton
All but one (Breton) are spoken in the UK. All have Wikipedias, mostly of a decent but small size (5000+ pages); Welsh is pretty flourishing.
Slight pedantry, the Isle of Man is not part of the UK, it is a crown dependency. The Constitution of Wikimedia UK does not appear to make any mention of the Crown Dependencies, so promoting the Manx Wikipedia might not be considered part of its remit.
I mention this, not because I think I wouldn't be a good thing to do (because I think would be), but because while WMUK awaits registration with the Charities Commission, we need to make sure everything that is done is within the charitable aims and I don't know if the Crown Dependencies would count as part of the UK for these purposes