Yes and no is the short answer.
Yes, you can get a fellowship. But you cannot gain subspecialty, or even general path board certification in the US, without redoing ALL of your path training in a US residency first. There's NO exceptions for the requirement to a) complete an approved US (or Canadian) residency and b) have a general board certification before you get subspec certification. From a US perspective, your MRCPath is, unfortunately, utterly worthless in terms of gaining specialist recognition.
The only US board that partially accepts foreign training is Int Med.
So you can apply for a fellowship in the US. In fact, you're probably likely to get it, because you don't need to fill a ACGME approved slot (since you can't sit for board examinations anyway). So essentially, it's training which will make you more valuable in the UK. It's not something which can be used towards starting a medical career in the US.
Oh, and remember to bring your own funding if you get a non-ACGME fellowship.