Ah now the WCO 'corridor' was different; it was higher (about FL200) and it ran just along the northern edge of Luton's airspace. It was established because traffic on the airway was either in level flight well above about FL245 or it was climbing out from or descending into the LTMA and thus below FL200 so there was this 'gap' just north of the LTMA which could be used easier than the DTY or LIC slots..