Ah, is this an Edinburgh 767? In which case that's the forward cabin and at this stage of the game it is reserved for Prems and Golds, of which the fair city of Edinburgh is amply provided. Less so for Glasgow but I don't think they block so many there. Nearer the time you should be allowed a bit nearer the front, however (a) at LHR you may be boarding / leaving by the back, and (b) row 13 is actually a very good option in the event you do not, in terms of ease of access and departure. There is no row 10 or 11, 12 is the bulkhead of the second cabin (and where I tend to head), so again a Gold zone.

You need to leave it until 10 days beforehand or so. Then 3 days (non exact) beforehand it's a free for all anyway.