I was thinking of responding about this, the unenforced BA size there is
40 x 30 x 15 cm
Whereas Ryanair is
40 x 25 x 20
easyJet is nearly twice the volume of BA:
45 x 36 x 20
Wizz is
40 x 30 x 20
Aer Lingus - not usually enforced:
33 x 25 x 20
jet2 - not usually enforced - is
deliberately unspecified dimensions, it just needs to fit under the seat (their fare and holiday package structure tends to avoid problems).

Now Ryanair and Jet2 are Boeing 737s with the others mainly Airbus 320s, but the seats are in essence the same and there shouldn't be a variance given all the verbiage by various airlines justifying different baggage policies. So we can assume that actually easyJet's dimensions are OK. I have a bag which is that size and it does indeed go under the seat in front. But it does suggest that maybe BA could increase the personal item size up to easyJet size in order to encourage people to bring smaller bags?