The 111s were fairly restricted at LBA although as you say BA did on occasions use LBA as a diversion fro Manchester. The secret of that was of course that the autumn and spring radiation fogs usually affected all the north of England airfields, whereas LBA at 680ft AMSL was usually sitting out, indeed VMC on top! Having diverted in, the BA aircraft would sometimes operate an outbound flight to Europe direct. However, such flights were usually part loads, from my observations. Probably half the pax cancelled rather than endure a two-hour coach ride from Manchester! I don’t recall Aer Lingus having any problems with either the 111 or the 737s to Dublin, of course a 30-minute sector. As with 27 at Jersey, the 15 take-off distance at LBA offered scope for longer flights/higher loads; sadly 14 departures could not be guaranteed.
I can’t say much about the Boeing 720s but in their executive ‘fits’ they probably had a pretty good range. Monarch did on at least one occasion use a 757 to the Med [direct] substituting for the usual 737. The departure was in the 33 direction and left a lot of unused runway!
As for the summer late evening flights to the sun, I do believe that temperature was a factor, the later the departure the lower the temperature. A technique not unusual at airfields with field length limitations