I would do (and have done) as you plan.

i.e. Spend 3½ hours in the T8 F/L before heading ove to T7 2 hours before your flight. (Note boarding will commence 30 - 40 minutes before the scheduled flight time).

Food wise, the catering is much better in the T8 F/L. (As posted it's unlikely you will have any access to the T7 Concorde Lounge.)