This is a fairly cheap way (though I can't guarantee it's the cheapest)
Super Off-Peak Return (London Midland Only) Watford Junction-Crewe £13.85 You'll typically have to use this on the 1401, 1501 or 1601 direct services from Watford Junction to Crewe on the Friday, as it's not valid until quite a bit later because of the evening peak.
You then need a Crewe-Lancaster Off-Peak Return costing £24.35
At short notice, and considering the good value of these above fares, it will be difficult to find cheaper Advance tickets.
No, we can't advise you of the cheapest ticket, as none of us can be sure what we have found will be the cheapest. We can however hopefully reduce the journey cost for you compared to what you have already found.
The cheapest through ticket is a Route Not Via LondonOff-Peak Return at £53.55, if you are returning within a month, which is valid at all times on Virgin Trains services.
Have you already found something cheaper or is this the cheapest you have found?
In addition, I think you have your travel dates muddled up a bit.
I remain reluctant to answer when such requests for the cheapest ticket are made, especially given that when the cheapest is offered, it's not uncommon that it turns out not to meet the passengers' needs.
Can you satisfy me that you've read my post #11 from one of your previous threads please? (It may be worth having another read).
Again, I can only really repeat what has been said in your other two threads, that a London Midland Only ticket as far as London Midland will take you (in this case Crewe or Liverpool), and then another ticket from there, is quite possibly the cheapest.
It will be cheaper to take London Midland beyond Crewe, all the way to Liverpool (a Super Off Peak Return is £24.00), and then obtain another ticket from Liverpool to Lancaster (an Anytime Return LIV-LAN is a very reasonable £17.20). It's a lot slower than a fast, direct train. But it is highly likely to match the criteria of the cheapest, if that really is the only requirement!