I have sympathy for anyone who is forced to tear up roots in one place and move to somewhere they wouldn't really choose to be in order to keep their job...
However!
When you sign a contract in which you agree that you, "...are liable for transfer to any part of the United Kingdom of Great Britain and Northern Ireland in the course of [your] employment." Then it's pretty clear that your employer is operating well within the terms of your contract to give you fair notice of a posting to another part of the UK.
On that basis, suggesting that you may have grounds for constructive dismissal based on an employer's breach of contract - when it is the employer who is applying the terms of the contract and you (theoretically) the employee who seeks to circumvent them... well can you really see a court supporting that?
That aside - there are some issues relating to the financial administration of a relocation in such difficult times. My first thought was that surely more people will seek to rent, rather than buy for the moment and wait on the market recovering.
I do know that there have been a few furrowed brows looking at the hit the company has taken on sales at reduced prices (under GSP)
I really think the guys at Manchester need to stop expending energy fighting the right of the company to move them - you ain't going to win - and concentrate on the detail of how the company works with you to ensure you are no worse off because they chose to move you at a time of recession.