What do Pruners consider would constitute constructive dismissal?
Now I'm certainly no lawyer, so what I say next is my opinion rather than legal opinion or advice. However, I think that hconstructive dismissal occurs if an individual resigns because they believe their position has been made untenable, and that the individual can prove that the employer acted illegally, breaching contract, in making the employee's position untenable.
Wiki seems to suggest that a unilateral contract change such as a dramatic change to duties or if the OP is told he is being posted to the other end of the country to do job Y, having signed up to do job X, that might be a breach in terms of flexibility as it is implied that the flexibility required of the employee relates to his original role.
All so far so good. However, surely the fact that we serve at HM pleasure, don't have a contract as such, and sign up knowing before hand that we are likely to get stitched up with an inconvenient posting at some point would probably make it difficult to argue constructive dismissal on contract grounds. I would have thought the only way you might get that one to stick is on procedural grounds or a technicality. Just my unqualified guess and speculation, could be totally wrong, so don't take it as gospel!