>>> I already have one property let out.
Then, as far as 'officialdom' (e.g. the tax man) is concerned, you're
already running a business. Irrespective of whether or not you decide to add additional houses to your property portfolio, you're free to decide whether you wish to continue operating your business as a 'sole trader' or to set up a limited company. (There's more paperwork with a limited company but it ensures that any losses that the company makes won't have to come from your own pocket).
Make sure that you're fully aware of all the relevant legislation though. These links should help:
https://www.gov.uk/renting-out-a-property
https://www.landlordzone.co.uk/
(That website is a vital resource for all landlords. It's well worth subscribing to it).