You can use openrents AST and do everything manually. You can also just use their referencing services, you dont have to commit to their full service. Yes they can pay rent direct and you can lodge deposit yourself.
There are plenty of other referencing services independent of openrent.
You should consider joining N R L A or other landlord organisation.
Yes you can have tenants pay you directly. Personally I use Openrent for all of my admin (contracts, deposit registration, referencing, inventory, everything except rent collection) as it saves me a lot of time and I use Openrent to upload various legally required documents, but I know lots of LLs who only use Openrent for advertising and then do everything else outside of the platform. So you can pick and choose to a certain extent which parts of the process you want to handle yourself vs doing it through Openrent.
Yes, I’ve been a landlord managing tenancies for 9 years, but always used a lettings agency to find tenants and draw up the tenancy agreement and referencing, so it’s that part I was asking about.