Income Tax question

[S6PNJ]

I'm in a fortunate position in that I have a small BTL portfolio and some additional savings. What I'd like to know is how can I best make my money work for me and make sure the Gov't get as little as possible by way of Income Tax? I complete an annual tax return so am not trying to do anything illegal or avoid paying tax that is rightfully due.

My BTL portfolio is mortgaged, so I currently offset the interest charged against my tax return.
My savings get me (say) 1% which I have to pay tax on (40%). For arguments sake say there is 80K in savings.
My mortgages are say 4.5%APR and lets for arguments sake say there are 80K left on each - qty not given!

So if I pay off a mortgage, I don't have any interest from that mortgage to offset against my Tax return, but then the mortgage is not costing me 4% (average) so I'm making 3% better return (notwithstanding the 40% interest or whatever it is I pay on my savings interest).

Is there some sort of equation I can use to work out at what point it is better to have savings vs mortgage or whether it is better to pay off lump sums from the mortgages and have no savings?

[Alpinestars]

This is a simple case of income versus expenses.

If A is the (gross) interest rate on savings
B is the (gross) interest rate on mortgage interest, and assuming you can offset it all against rental income
C is your effective tax rate eg 0.4 (40%),
D is the amount (80k in your example), then

Net income equals D*A*(1-C)
Net expenses equals D*B*(1-C).

This ends up with the conclusion that if your gross interest rate on savings (A) is higher than your gross interest rate on the mortgage (B), you'll end up in a net expense position, therefore pay the mortgage off.