Yes, gliders are allowed in IMC.
I probably did a similiar journey last week, in similiar conditions at a similiar time.
Birmingham will indeed provide an excellent service. Brise never close and you still need to ask for a transit when their LARS is closed. Easy to be caught out with this.
The problem with trips OCAS, or being dumped, is maintaining above the cloud. Often you will be forced down by the base of CAS into IMC otherwise you obvioulsy stay on top until your destination and at least hope for some protection in their procedure or if letting down without a procedure through good knowledge of where any gliders might be.
My own view these days is to avoid IMC outside CAS unless there is no alternative. Coming home it was interesting to hear the CAT asking for plenty of heading changes to avoid weather whilst maintaining below the base and skirting around the heaviest down poors with the strikefinder doing its job.
The trouble with this country is increasingly everything seems to shut around 18-00 - you can forget getting much of a service from many of the LARS units up narth. I suppose the only good thing is most people also seem to stop flying around that time so the traffic load is also down.
Bring on the day when gliders are required to carry a transponder if flying in or near cloud - non transponder should be for VMC, well clear of cloud, - hat coat etc.