National Rail Question

Spring is when lots of engineering gets done ,often weekends and especially Bank Holidays.
On a route like that ,there will be an easy substitute route ,if not replacement buses are often supplier by the rail company to jump the works .
I'm d be surprised if it's not updated within the next week or so

The particular route you're looking at is served by Avanti West Coast -- https://www.avantiwestcoast.co.uk/

It looks like they've not released any availability yet for any Saturdays/Sundays in March.
And their planned alterations for March-- https://www.avantiwestcoast.co.uk/travel-information/plan-your-journey/planned-engineering-works?year=2023&month=3

You can set up a ticket alert for your travel date on the Avanti website.

Engineering works--work on the tracks, sigaling, etc--is usually done on weekends. March 18 is a Saturday.

There should be service--hopefully--it just may be rerouted. Nothing to do with strikes.

EDITED- National Express now back on line-
Avanti are currently opening for bookings every weekend very late due to a combination of engineering work and crew rostering problems. Currently they are open to the end of February which is a distinct improvement on where things have been. Some weekends they have been opening as late as 48 hours beforehand. You just need to be patient. You may have a further 3 weeks or more to wait.
I have just checked London North Western Railway who are the alternative carrier on the route over the same tracks. Anyone who hasn't travelled on them may not be aware how nice and comfortable their long distance trains are. They are a good alternative to Avanti, and usually almost the same length trains, just no refreshments.
They are open for bookings as far as Crewe- which is 3/4 of the way to Liverpool, so there are no engineering works south of there.
Their trains take 2 hours 10 minutes rather than Avanti 1 hr 30 minutes and they currently have good availability at £9.50 each from London to Crewe.
If Crewe to Liverpool turns out to be closed that will be replaced by bus. Be aware that at Crewe it is quite a long walk to the side of the station around the old disused platforms to reach the buses (signposted). LNWR and Avanti both run from Crewe to Liverpool.
So you could book for now to Crewe, then buy a further ticket later or even on the spot for Crewe to Liv at £15.80.
The other route from Crewe is train to Chester, change for frequent Merseyrail train to Liverpool- same fare. One or other route will be open.
Or if you just want to be sure of having certainty or don't want to potentially mess about with changes book National Express, Megabus or Flixbus coach.
It takes 5 to 7 hours. NatEx website is currently down for maintenance but Megabus are currently £16.50 and Flixbus £12. NatEx will be similar. National Express are now back on line- they have several direct buses also from £12.
If you sign up for a free account with NatEx you avoid the booking fee as well.

According to the National Rail website under London and NorthWestern, some of the lines between Euston and Cheddington will be closed affecting their services. These changes are not currently shown on journey planners.
Nothing is shown against Avanti

The issue with London Northwestern Railway and Cheddington mentioned above will not affect anybody or delay any trains except those individuals who are only travelling between London and Bushey (south of Watford) on two late night trains.

From the LNWR planned engineering works calendar:

Saturday, March 18, 2023

Engineering work is taking place between London Euston and Cheddington, closing some lines.

As a result the 22:37 and 23:30 London Euston to Northampton services will stop at Bushey for customers to disembark only.

Having local knowledge, and having worked those trains and lines I can tell you that the West Coast Main Line is a 4 track railway from London to Wolverton just north of Milton Keynes. That means that a pair of tracks (one northbound, one southbound) is for fast trains with a higher speed limit and few stops, and a pair is for slow trains with all stops possible. That night the slow lines will be closed at the time of those two trains from at least Wembley (probably Euston) all the way to Ledburn Junction between Cheddington and Leighton Buzzard (south of Milton Keynes). All traffic will divert to the fast lines for the duration of that closure. Because Bushey station on the fast lines is not a normal calling station, when trains are diverted onto the fast lines there it can only be used for people leaving the train to exit the station through a special door and no passengers are allowed to board the train. Standard operating procedure there. Of no consequence to anybody else.

It does serve to warn commuters on those two trains that they will be using the unusually used fast line platforms at Harrow and Wealdstone, Bushey, Watford Junction, Kings Langley, Apsley, Hemel Hempstead, Berkhamsted, Tring, and Cheddington.

Although LNWR go through to Liverpool, the scheduling for the northern half of the route is maintained by the sister company West Midlands Railway (both owned by West Midlands Trains). So off to their website for their planned engineering works for that day.

The only thing I find for that date on West Midlands Railway is

Amended services between Shrewsbury and Birmingham New Street from Saturday 18 to Monday 20 March
West Midlands Railway between Shrewsbury and Birmingham New Street

So nothing there.

I expect that it is only that as usual Avanti West Coast are slow to pull their finger out. Be patient.

Even if they go phut you can still get there with LNWR and a bit of patience.

Hope the extra detail is of some interest even if of no use.

Thank you so much for all the helpful feedback. I am so appreciative, especially to the locals for providing such excellent guidance and interesting feedback!