I would probably agree with Giles. The descent planning profile for the LAM 3A STAR also has expected levels for LOGAN, so I would expect your descent to conform with these too.
So, descent at your discretion to cross LOGAN at FL250/220, continuing to cross SABER at FL150. Maintain until LAM, then descend in the hold.
Two points to note:
1. If you're going into Heathrow squawking 7600, there ain't going to be anything in your way! You could descend practically anywhere - it would be a rather foolish ATCO that positioned any other traffic anywhere near you.
2. In the current political climate you may not find yourself alone for long - does anyone else think that the RAF's finest might be checking you out prior to your arrival in London, given that a radio failure could indicate a myriad of on-board scenarios?
Back to the point... yes, if your STAR has a descent planning profile, then we ATCOs would probably expect you to follow it. If there is no published profile (unusual for LTMA airfields, but may happen further afield), the book says you should arrive at the IAF at the Minimum Holding Level (7000' on your best guess QNH, if this procedure applied at LAM?)
Essentially, the book is written by a bunch of bureaucrats who don't do the job it was written for, so some things written in there are not always practicable or even sensible (I'm not necessarily saying this about the radio failure procedures). Suffice to say that if you're completely radio-fail, you'll descend and land at Heathrow in a way that more-or-less conforms to the procedures, and we'll get everything out of your way. Common sense will prevail.
LTP