At the risk of thread drift, and this is pure armchair/keyboard educated guessing
If you take the altitudes from those three sites and adjust for 30 feet per milibar difference from 1013, that gives you a pretty accurate picture of what Altitude above AMSL an aircraft was flying.
eg if the sites say the "barometric alt" was 1,200 and local QNH on the day was 1006 mb...
1013 minus 1006 = 7 mb
7 x 30 feet = 210 feet
1,200 minus 210 = equals Aircraft was 990 feet AMSL
Elevation of Northern Doncaster is about 70 ft AMSL
900 minus 70 = 930 AGL
The average two storey builiding is 20 feet high
930 minus 20 = 910 feet
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SERA 5005:(f) Except when necessary for take-off or landing, or except by permission from the competent authority, a VFR flight shall not be flown:
(1) over the congested areas of cities, towns or settlements or over an open-air assembly of persons at a height less than 300 m (1000 ft) above the highest obstacle within a radius of 600 m from the aircraft;