Bear in mind when connecting at LGW I believe Monarch operate from South Terminal and BA from North so if not a through ticket 90 mins connection time on the return could be tight.
Monarch operate A330 non-stop LGW to MLE on Sunday evenings. As already mentioned BA will do the same on Tues/Fri/Sun with B777 starting this winter. Thomson also fly non-stop on Fri and Sun evenings this winter with B767-300.
Emirates from MAN via DXB may be just as easy but check the return departure time from MLE as sometimes it is early morning (0200ish) which could result in having to book hotel room at MLE at extra expense.
Monarch web site gives details of the premium cabin product. Not sure of exact cost of upgrade but around £200-£250 pp may apply.