If you do not wish to proceed, simply do not sign the finance agreement or hand over your old car, tell them you do not wish to proceed. Do not feel pressured into committing yourself to a new re-negotiated deal. at least until you have had the full opportunity to go away and think seriously whether it's the deal for you.
You are currently not legally obliged to proceed with the deal, as the finance agreement has neither been signed by yourself, or signed and accepted at the other end by the authorised signatory at the finance company, if for any reason the finance agreement for a credit dependent purchase does not proceed... then the whole deal is cancelled.
If you do sign a credit agreement never sign it on the dealers licenced premises, that removes your rights to a cooling off period............always take it home and tell them you need to think about it. !