I am one-of-the-others who used the sale of our UK home as assets...
I'm the USC, hubby the Brit. I'm a full time house wife and hubby did not have a permanent job to go to. And, I was the sole sponsor (no joint sponsor) so I sponsored DH strictly on our assets, which primarily was the sale of our house.
If you look on the instructions for the affadavit of support, it will give you information on what kind of assets can be used. You can use the proceeds of the sale of your UK house.
Our house went on the market in April 2004, simultaneous with filing our 1-130. By the time we had our interview at the Embassy in August, our house was under contract, with a completion date set just 10 days after the interview. Also, very important: you need the written proof from land registry that you are the rightful owners of your property and have the legal right to sell it!
On our interview day, we brought a copy of the contract of sale, land registry docs and letters from our solicitor about the completion, to show that it was all valid. He was very happy with this and didn't even question it.
However, if you were planning to use the proceeds of the house and it was not yet sold, I believe that per the I-864 instructions, they would require a written valuation from an estate agent to prove the value of your house. (More than one valuation probably would not hurt...)
I hope this helps. Let me know if you have more questions.
Please note that the rules are changing constantly. Always double check information on the Embassy website.