Ella and family on the 1841 census:
At St. Peter in the East, Oxford
William Thompson - 30
Ella Thompson - 30
Walter Thompson - 4
William Thompson - 2
Anne Thompson - 1
All are shown as born in the county. William's occupation is hard to read - but could be "print seller" (willing to be corrected on this!).
And Ella's parents on the 1841 census:
At Littlemore, Iffley
Richard Allin - 60 - Farmer
Ann Allin - 60
Lydia Widdows - 25 - FS
William Allin - 25 (occupation illegible)
George Allin - 30 - Tallow (Chandler?)
Elizabeth Allin - 30
Louise Allin - 6
All born in Oxfordshire, except George.
From the OFHS's Oxfordshire Marriage Index (1538-1837), this could be Richard and Ann's marriage:
30 MAR 1795 - Richard ALLEN to Ann BROWN at Piddington
Note that ALLEN is spelt with an "E". The marriage is a bit early date-wise, but there could have been elder children who would already have left home by the time of the first census.
Littlemore/Iffley is on the county boundary of the old Berkshire parishes (inc. in Oxfordshire since 1974) - so it might be worth asking the OFHS for a lookup of their N. Berks MI.
Piddington is in the NE of the county, on the boundary with Buckinghamshire (your reference to High Wycombe?).
Hope this helps!
Pennie
PS 1851 census gives Ella's birthplace as High Wycombe - both Piddington and Littlemore/Iffley are approx. 25/30 miles from this town!