Points in his favour:

1. The work of the High Rep will have little to do with Schengen and the euro, so UK opt-outs in those areas are not problematic in the same way as they are for candidates for European Council president.
2. Although a member of the Commission, the high rep will be out of town much of the time (and will have plenty to occupy them when they're not). So it will be harder for him/her and their cabinet to influence dossiers in other areas of Commission work, such as the economic/single market/competition fields. Now wouldn't it be attractive to France and Germany to keep British fingers well out of those pies?

Miliband, Flavour of the Day - afew