This is actually quite common practice. Not only on standard airmail (where customs will likely be more lenient, sometimes not even checking package values), but also with couriers like DHL/FedEx etc., where more higher-value packages will be sent.
With the major couriers, customs will usually receive electronic copies of the commercial invoices before the plane has landed. This means that, in most cases, packages will be cleared before even landing in the UK (providing they pass the usual security checks on arrival). So there's very little manual checking. Additionally, couriers are working to deadlines, so can't leave packages backlogging for days waiting to be cleared.
With the volume of incoming non-EU mail the UK receives, I doubt there's the resources to do checks on standard airmail packages, unless sent by more expensive methods. There are simply more important matters.