Also bare in mind that if your purchasing a vehicle already registered in a foreign name, the seller needs a residency certificate too to be able to sell the vehicle.
I'm not entirely sure what type of pension you are referring too? If your in receipt of a state pension of being 65 years old or older, simply apply for an O-A visa in London on that basis. The UK is the only country that will issue this type visa on that basis and only applies to UK citizens. If your applying on the basis of being over 50 years of age but under 65 and not drawing a state but private pension, you will also need written proof of income from your pension provider. Signing a declaration only doesn't work for UK citizens and won't be accepted.
. As Tod has answered more than once. You COULD be asked for proof of funds which needs to be in cash or travellers cheques. Having a return flight and hotel confirmation is a totally seperate requirement that if you never had you COULD be refused entry for if immigration required you to prove and you never had it.
So too answer your question, if your required to show the funds and you don't have it in cash or travellers cheques, a refusal of entry is highly likely and a return flight and accomodation, won't save your day.