The disadvantages of an arrangement such as you suggest lie with Sussex and Surrey Police. At present, each has high quality air support provided by a helicopter that operates in a relatively small area. Response times will be good, so allowing the Units to intervene rapidly, with associated high success rates.
If a police hele can respond to the scene of an incident within a few minutes, it has a much hgher chance of successful intervention. If it take 30 minutes to transit, many reactive jobs will have finished before the aircraft gets there.
If you use the same aircraft to cover a MUCH bigger area, then the benefit to Surrey and Sussex of their air support will be much reduced. Their council tax payers have paid a significant sum for their Air Support. It would IMHO, be a great shame to have that diluted, so that Kent could be said to have cover on the cheap.
Surrey has past experience of "sharing" air support with a larger partner. If you were to ask those who were involved, I suspect they would advise you of the many disadvantages.