It’s a good time to buy while a majority of property is bumping along at the bottom of the market.
Most BTL lenders are still looking for 30%-40% deposit so it’s worth finding out what mortgage products are available to you before considering what percentage of your current property to re-mortgage.
Good plan to sell up in 30 years (or maybe less depending on market performance!), worth doing some planning to minimise your capital gains tax liability at your exit point.
Must point out that I’m not a financial advisor but I am a BTL landlord!