The disability assistance service at Heathrow is so poor that we avoid it whenever possible and I now fly with my mum (at my own expense) because I don't trust Omniserve to do their job properly.
First off we have been told they will only meet people in a specially designated area (zone H?) which is the opposite side of the terminal from where we normally arrive by Tube. To be honest the walk through security, to Galleries North to a low A Gate for a domestic flight isn't much longer than this so we normally don't bother.
Secondly we've met at least two Omniserve agents who (while very nice) say they can't push wheelchairs uphill (i.e. up air bridges) since some of the domestic air bridges are improbably long and this is where we really need the assistance it sort of defeats the point.
In Heathrow's defence though it is far better than Paris CDG. I'm fairly certain we'd still be sitting there now if I hadn't gone and spoken to the desk in French at least 8 times (we only made it since they screwed up the boarding and let a load of EDI pax on a GLA plane and vice-versa). What disabled travellers who don't speak the language of the airport do I don't know.