Fixes plausibility? for variable length numbers from the United Kingdom

Fixes 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
how it should be according to the data available at
http://www.ofcom.org.uk/static/numbering/index.htm which only lists 4+6
as the format for allocated numbers.
Merge pull request #196 from Emily/fix_variable_length_uk_number_plausibility

Fixes plausibility? for variable length numbers from the United Kingdom