The agent is wrong, that's an M fare (probably MK5). On travelocity, they list out as:

V: approx $1600
M: approx $3100
B: aprrox $3400 (worth it if trying to upgd!)

If you are not so worried about upgrading, then I suggest trying someone like ebookers.com. For departing 9 sept, return 27 sept, it spat out 750 quid (tax incl).