Now, that's agents for you and they are not great at thinking outside the box, they are not investors or renovators so they tend to be terrified of things they don't understand or have any concept about.
Ouch that hurts! Not all agents suck!
Cashflow Plus get yourself a good solicitor that understands what you need, be prepared and have the clauses written out for you prior to putting in an offer on a property, then give the clause to the real estate agent and ask them to add it to the special conditions. Make sure that everything is upfront when making the offer because once the offer has been agreed to then the real estate agent really can not help you to make adjustments to the contract. Any and all changes that you make to an already agreed to contract will need to be done through your solicitor and the sellers solicitor. Don't rely on real estate agents to write your clause for you because they do not have any idea of what to write and a good solicitor may be able to cancel the contract based on a poorly written real estate agent clause if the seller wanted to crash the contract for whatever reason.
If the sellers don't allow full access to renovate the property prior to settlement then request/negotiate several access visits in order to get quotes, at least you'll be able to get the ball rolling and be able to start the renos as soon as the property settles.
Once again get the right solicitor and get the clauses right from the beginning so that everyone knows where they stand and there will be no hiccups later and above all make sure that you are protected from any legal issues.
Good luck