You are currently preparing to move to the U.K..

Whether it is a permanent move or to take up an assignment, one of the many important steps is negotiating your compensation package. 

Yes there are tools that can spit out the cost of living differential and your estimated taxes but they do not take into account your complete set of circumstances or any upcoming changes that may have a significant impact on your tax position, in the short term and long term. 

Seek counsel during the planning phase, to better anticipate and manage upcoming costs.