NorthSouth its a nice idea but it has a few flaws. With your AFISO service you seriously limit your traffic, i don't know what SirP wants but maybe it would work for him, your other problems would be if he wants ILS, NDB instrument approaches to keep your training schools happen, if you want to maintain the buisness your going to need those because Coventry is well known for them. You can't offer those with just an AFISO service all the time.
As for farming radar off to Birmingham, similarly it won't happen. NATS have in the past told controllers at Birmingham not to work Coventry traffic because they arn't getting paid for it and they arn't interested, so long as no one enters their airspace they don't care (company viewpoint). They also don't want their controllers being distracted from the work they are actually being paid for which is their own traffic at Birmingham, nor are Birmingham airport going to want to be paying for air traffic control services for Coventry. Yes controllers being controllers are trying to help everyone but they certinatlly arn't going to be offering radar services to everyone and it won't be consistent, when they are busy they will not offer anything to anyone other than what they are paid for. That is unless they want to agree a contract with NATS to provide Coventry radar, which would obviously come at significant cost as NATS would have to get another few controllers in just to work Coventry.
Which leads you all the way back to if your going to be running your own Tower then you might as well run your own radar...
Also unless proceedures change another radar unit can't vector someone onto another airfield's ILS as they have no idea of what state the ILS is in that they are vectoring onto, which means you have to have an agreed procedure, if you do agree a proceedure then your setting it in stone that someone is going to have to work your traffic and hence the money gets back involved.