First visit by a 707 was on 25/1/70 when G-AWWD of Caledonian diverted in en route Tenerife-Gatwick. The next one that I know of was Lakers G-AVZZ on 3/3/70 which was the smaller -138B variety. This was on a night when Heathrow fire crews were on strike. I believe the first Midland example was G-AYBJ on Sunday 31/5/70 which I only saw from home as it took off. Unless anyone can supply info on other 707 visits, I believe the Britannia G-AYSI on delivery would have been the next one on 28/2/71. Attached are some relevant newspapers cuttings.
Obviously the Dan Air Comets referred to above can't have been the usual 4-engined variety!
Once Britannia began operating their 707 other examples suddenly became almost commonplace, with the first 2 Donaldson aircraft G-AYVG and G-AYXR appearing for maintenance in April and May 1971. Midland operated G-AYVE on Easter Monday 12th April '71. The European Cup Final on 3/6/71 brought in Transavia and Air Commerz examples, then Pan American no less brought in N766PA on 25/7/71, operating a Britannia sub.