Firstly, your income form a UK company and I assume being paid from the UK is only tax free as long as you do not spend more than 90 days a tax year in the UK. If you spend between 90 to 120 days you come under the significant ties questions, one of which is "do you have accommodation readily available for you to stay in the UK", which in your case is yes. So, firstly assuming you do not meet the first condition, it may e better to rent the whole property. If you fall foul of these rules you will be liable for UK tax on your whole income for that tax year.
All income from rentals will be subject to UK tax regardless of your tax status because it is income received in the UK from a UK asset. You can of course offset mortgage interest and repair costs as well as the cost of compete re-decoration upon your return assuming you kick out the tennants and live there yourself. These details will be entered on your tax return which must be filed as you are, I assume, paid from the UK. So, rental income is going to take a very long time to provide the next BTL deposit!
The BTL market is pretty much sown up, you will make around £200-250/month gross profit based on 25% deposit and current mortgage/rental rates regardless of size/value of property. Hence a proliferation of gurus, consultants, trainers etc offering the secret to property success. It's property for God's sake and you've already bought one!!
The trick in property, for which you can pay so called consultant training, er Simon Zutshi style for instance (£500 one day - special offers available - £1400 weekend or £14,000 for a year mentor-ship - can you believe?), is to buy property in stress (at or near repossession/auction) or in need of refurbishment and therefore below full market value. Do it up, revalue and remortgage and there's your next deposit - assuming no property crash. There are now restrictions imposed by mortgage companies that mean you have to keep the original BTL mortgage for a minimum period, typically 6 months.
In your case you will better off to rent your complete existing property and save your earnings for the deposit for your next/first BTL. BTL mortgages rely on rental income and not your affordability to pay the mortgage. They do however require a 25% deposit - maybe 20% if you're lucky. But do make sure your tax situation is rock solid or that deposit may be taken in the form of a tax demand!!!!
For your original question, BTL landlords generally remortgage their portfolio properties (or main residence) to raise deposits for the additions. As long as house prices increase, or you add value (extend/refurb) this works. But as you can see, it does have "house of cards" tendencies in the early years, so be careful.
You will also see a lot of no money down marketing blurb form these so called consultants (er no professional qualification by the way) and this is as a result of the recent property slump and serves no purpose other than to take away your savings which would be better used for your deposit. It is aimed at people who really can't afford it in the first place, selling them a dream! Additionally, what most of these so called "specialists" forget to mention are the true costs involved. The nett returns are punitive with the true value being gained from capital appreciation over time.
You have time on your side, so good luck!