^^^^ Aagh!!! No! No! No! Do NOT use The Trainline! They charge a 'service fee' for each transaction, which can be avoided by going through the websites of individual train operating companies.
Further, they've been known to issue tickets that simply aren't valid for travel on any train! (When I ran a railway station, I got fed up of seeing tickets from The Trainline for, say, a service from operator A but with a ticket that was only valid on the trains of operator B, providing numbered seat reservations despite the fact that the train in the booking didn't offer reservable seats).
It can also be harder to get a refund from The Trainline, if things go wrong, than it can be to get one from the relevant train operator.
Search for tickets on the National Rail website:
https://www.nationalrail.co.uk/
(Although National Rail doesn't have its own booking service, you can click through from your search results to a train operator's own website, thus enabling you to avoid The Trainline's booking fees).
As Barry indicates though, it's
sometimes possible to get cheaper fares by booking a journey in two stages. (i.e. instead of booking from A to C, book from A to B and then from B to C). 'Ticket splitting' websites can help in the search for ways to do that but they're far from infallible. (I've several times managed to beat their offers by searching manually for fares).
If you tell us
(a) when you need to travel . (Date and time, please ) ;
(b) your return travel requirements, if any ; and
(c) what flexibility there is in your plans ;
the people here who regard rail ticketing as a specialist field (such as Mushroom25, Dzug2 - if he's still around - and myself) can take a look to see what might be available for you.
Note though that it's usually pointless trying to find discounted fares at peak travel times, such as Friday evenings, on major inter-city routes. You're usually stuck with having to pay the full 'walk-up' fares.