HMRC didn’t “take a dim view of it.” HMRC agreed it.
I am told that the local HMRC office agreed to the bond scheme in the distant past, but the continuing agreement was contingent on there being a reasonable take-up from airlines 'buying' the bond and then discharging it through an element of pay. When that ceased to be the case, but the bond scheme continued anyway, it raised questions from different sections of HMRC about VAT avoidance, as the primary benefit to the ATO seems to have been that no VAT was paid on a large element of the training costs.