AllanJ - most London 'minicabs' operate without meters, on a negotiated fare. Only the black cabs are metered. This varies place to place...

In major airports, it's pretty rare for illegal cabs to get to the official taxi rank, but then that depends on the airport. Places like PRG and BUD don't really have official taxi ranks, but rather 'Dispatchers' for private companies. LHR and LCY are the only airports in Greater London, so are restricted to 'licenced hackney carriages'.

And, of course, how valuable this is depends on local legislation for taxi licencing. If you're effectively allowed to stick a taxi sign on your roof and take fares, then you're not 'illegal', but how reliable and safe are you?