Fixes plausibility? for variable length numbers from the United Kingdom

Fix for https://github.com/floere/phony/issues/169

Plausibility is now true for a number of the form `+44 1827 XXXXX`. However, plausibility is still false for `+44 1928 XXXXX` (the other number listed in #169), but that is correct according to the data available at http://www.ofcom.org.uk/static/numbering/index.htm which only lists 4+6 for 1928.