You don't say when the wedding is to take place.
Might it be worth asking the relevant Register Offices if they have smaller venues in the pipeline waiting to receive their licences?
Also, the Register Office may be able to tell you if any smaller venues are registered with them that are not publicised as much as the larger ones. You'll find they'll be very helpful.
Alternately if you have a few venues you like the look of, pluck up courage to ask the manager if it would be worth his while considering applying for a licence. It might easily be too expensive but you may set them thinking about it.