The tax calcs look right (roughly, as you say)
You will lose TRS but whether that makes your interest payments go up will depend on the mechanism the bank uses to apply TRS to your account. For example, in my case, the TRS is added to my current account, from which the mortgage is paid. The mortgage payment stays the same (interest and capital) but my current account amount increases by the TRS each month. If I lost TRS, then the payments would still stay the same (disregarding rate changes) but my current account wouldn't get the monthly increment. I don't know if that's the way it's usually done but you should check with the bank. You will be down by the TRS amount each month in any case, whatever way the bank calculates it.
Another wrinkle is that the bank may change your rate if you change from a PPR mortgage to a BTL mortgage (they were going to change my rate when I was thinking of renting out).
I still don't understand how the interest portion of the mortgage is so high as compared to the capital portion... My mortage is about €190k and I've been paying for 6 years - my repayments are about €900pm, made up of about €600 interest and €300 capital.