You must purchase a separate ticket from the Plough roundabout, Crossens, the most northern stop in Merseyside for onward travel to Preston as you'll be out of Merseyside.
conversely off Stage coach Lancashire take up the government's £2 fare cap scheme it may only end up costing £1 each way off you are under 16 years old.
Even though Merseytravel extended the age of child fares to the 19th birthday a few years ago, fares in Lancashire in most routes are up to 16 years old.
Stagecoach do, however, have their own Explorer ticket which is an extended Megarider which is very reasonably priced. IIRC an adult can travel all the way from Chester to Annan in the Scottish Borders for round £12 and a child under 16 for £9. Absolutely major value but one back of a journey, mind you. You could have trip from Liverpool to the Lakes reasonably in a day-to-day its a bit of a haul though.
I hope this helps you.