It's mobile grade policy and it's application,
Has been challenged at employment tribunal, albeit some time ago, and found to be entirely legal provided the company's reasons for the move are "reasonable". Although it was a long time ago (mid 80s I think) the relevant employment legislation has hardly changed since then.
As to the rest well most UK legislation is almost deliberately ambiguous and the same tests of reasonable tend to apply. Most avenues of challenge are either expensive or require you to resign or be dismissed before they apply. Given the poor record of Tribunals on defending employee rights and the small amount of redress normally available it is not surprising that most people put up with it rather than resigning in order to become the test case.