At the link a pdf with a plain English (hopefully) discussion of the fact that if you continually use mortgage equity withdrawal to extract the capital gain from a buy-to-let, the answer to the question in the thread title is that in the end HMRC own your buy-to-let, and if you are really incautious you may create a situation where they end up having a claim on your other assets too.
Who owns your buy-to-let?
Please feel free to tweet, e-mail or print it off and leave it surreptitiously in the path of anyone who is using mortgage equity withdrawal to treat a buy-to-let as a piggy bank. I've never dealt with a mortgage broker, but neither have I ever heard of one warning about the interaction between mortgage equity withdrawal and Capital Gains Tax.
If you want to improve it or change it PM me and I'll give you a link to the same document as .rtf or .doc according to your preference.
Enjoy.