I just returned on Monday through Eurostar UK security at Brussels and yet again my passport failed at the E-gates (0 of 6 attempts in the last year successful, mostly T5 LHR).
One station manned with an indian lady with about six children standing at it, some of the children were were not related, queue behind her I'd probably still be there, if the second desk didn't open.
At least at LHR there's a desk to deal with the system failing (and I've never had e-gates fail before my new passport in the UK), but a failing system needs contingency to cover people the parts being broken. I think they understand that at least at T5, never waited more than a few minutes to get through,
Though I must admit, they've had a goooood look at the passport there, and the last few times through T5. I have a common name,