Just to clarify Max Angle's statement. There is indeed an FCOM3 procedure for single engine taxi out but the bmi company proceedure is a slightly modified (& approved by Airbus) version of this.
The only difference is we do not open the cross bleed after start (pack 2 is normally switched off at this point anyway in the bmi SOP).
There are a number of other airlines employing reduced engine taxi out procedures at LHR. Notably Etihad use a 2 engine proceedure on their A340s.
There is arguably a higher chance of a tail pipe fire on shut down but no one seems to worry too much about shutting an engine down during taxi in!