Don't think Monarch had a trans-atlantic operation when they bought the 720 - I think they got them cos they were cheap. Remember we are going back to the days of affinity charters etc. The 720 doesn't really have that sort of range and don't remember Monarch having the 707 as a mainline a/c (might have been some short term leases?).
The issue with the 727 in the UK was the amount of work (and money!) required to get them approved by the CAA (stick shakers?)
As an aside, was working a MON 720 from MAN-TLV which was over-booked and I remember walking down the queue of people saying "do you want a jump seat in the cockpit or we will have to leave somebody behind..." - there were no takers and we ended up bumping a family....