I don't think that anyone's going to be able to put a number on it for you. There's no question that it will be much more difficult to do so. Plus, some programs might be willing to consider your UK experience as "good enough" for them.
You could try to research programs. Look at their websites, try to figure out how muc USCE is needed. It may be listed on their websites, it may not. You may need to email/call them to find out, and you might get a vague answer like "we review all applications on their individual merits". This could be a tremndous amount of work.
You could buy a list off the internet. In the past, I've seen people selling lists like this -- presumably they did the work, and now are trying to profit off of it. It's not known how up to date these lists are, so buyer beware.
Or, you could just apply broadly and see what happens. Programs with USCE minimums would simply reject you. This is less work, but more expensive.
Perhaps someone with a similar background will post here.
I applied with 2 LORs from Australia and 1 from the US. Most of the programs didn't even notice that my other two were from Australia. As long as they're well-written in normal Western format by a person with an English-sounding name, many people in the less-competitive fields (i.e. FM) probably won't realize that they're not from the US.
I don't know if this made any difference, but I made it a point to ask for rec letters from people with 100% Western-sounding first and last names (i.e. McSomething, etc.). Largely because I have a foreign-sounding name, and I was wanted to make sure to minimize any assumptions that my letters were written by anything other than academic faculty at a well-respected institution in a developed country. It probably didn't make any difference, but I like to believe that it played into people's tendency to automatically create stereotypes and made them think "oh, this letter is from a guy named McSomething who is an Associate Professor of internal medicine, so I know what that means"...