Domestic trains in UK apart from sleeper trains do not require reservations, you can board any train with your pass and take any unoccupied or unreserved seat (seat reservations are displayed above seats on those trains that have them).
If you wish to reserve seats you can do that for free at any manned UK station or online at www.lner.co.uk for that company only or by registering an account at www.gwr.com for all train companies.
I just reserved seats through www.gwr.com! does the global interrail pas count as the tickets in this case? Or do I need to buy something extra?
Your pass is the ticket which is valid for travel, any reservations you get are to accompany that. In the case of countries/services where reservations are non-compulsory such as UK, all they are for is to give you priority over the marked seat.
You may notice that often reserved seats are not occupied, this is not unusual for various reasons. It is also the case that if you don’t like your allocated seats you are not required to stay in them, you can move to any other unreserved seats if you prefer them.