I agree with the other posters that you seem to know what you're doing and that the decision may depend partly on whether you can invest sufficiently using your ISA/JISA and SIPP allowances while also having your BTL.
One approach might be to make 2 plans, one to keep and one to sell.
If you sell, what would you have left after CGT and costs, and how would you invest the proceeds?
If you don't sell, what would your plan be?
You've been thinking about your strategy already, if you write down the two options in detail that might help you decide
My wife and I have 3 BTLs, my approach is to go for 5 year fixes and always try to maximise additional borrowing each time a remortgage comes up (borrowing up to 75% LTV max, as that gets decent rates). Whenever I can get additional borrowing, I channel that into ISAs and maxing my pension contributions.
That has worked OK so far, at least it does in a bull market :-)
For higher rate taxpayers, nowadays you can end up paying lots of tax on your BTL, even if you make a net annual loss. One early mistake I made was having ownership in either my name or jointly, so I am thinking about changing this so that my wife (who doesn't work) gets notionally more BTL income. This will be more tax efficient, but there's stamp duty costs (I think 3%) involved in transferring part of ownership, even to spouse.