Is it possible to watch a debate in the House of Commons?
Yes, but it is advisable to apply for tickets in advance as otherwise it can often involve a long wait (often 1 or 2 hours). Details of how to apply for tickets are given on the Houses of Parliament web site.
If you are a West Edinburgh constituent, you can apply to Johns Parliamentary office with a request for tickets. They will do what they can to meet such requests, but please note that MPs have only a small allocation of tickets, and by no means for every day, so demand often exceeds availability.
Please also note that it is very difficult to get a ticket for Question Time, especially Prime Ministers Questions. Question Time takes place on Mondays Tuesdays and Wednesdays from 14.30 hours to 15.30 (with the Prime Minister answering from 15.00 to 15.30 on Wednesdays) and on Thursdays from 11.30 to 12.30.