Best thing to do RedMonster is to phone up HR. Their number will be on the emails you have received..
Dates come up at the Glasgow Hilton on a regular basis (Thats where I had my S1 and S3). They may not be able to confirm any dates until they are published on the booker, however they should be able to tell you if Glasgow is still a viable option and they may make a note on your application stating that you are waiting for local dates to become available.
That is, of course, if you were really looking for a Scottish date. If you have no problem in spending your cash to get to the assessment then I would like to be your pub friend. My phone number is......
Andy