UK last train cancellation and unhelpful staff – what to do?

First, get to your destination. If the only practical method is a taxi, get a taxi. Keep the receipt.

The next day, start working on getting a refund. You will, eventually.

If you are in the situation, train staff not helpful, late at night, no trains to get you anywhere, buses stopped for the night and only taxis to get you where you need to go, you have few options.

1. Take a taxi (or call for one if you need to.)
2. Stay in/near the station till the service starts again.
3. Call a friend or relative to come and collect you.
4. Call the police.

In most cases options 2 and 3 are either not acceptable or possible, so only option 1 and 4 stay.
More on option 4 later.

If you take a taxi for which you have to pay yourself, ask for and keep the receipt and contact the train company the next day to ask for the money.

Their site has several options for contact.
I feel it is up to you and not a stranger who has not has never used their trains to contact them to find out what they think you should have done.

Whether you call the police, option 4, depends mostly on the police in the area and whether they would be helpful.
I have been brought up with ‘the police is your best friend’ and I would call the police for advice or even help if the other options are not available for me. But I am aware that it can be a no go in some countries/areas and as I do not know which station you were abandoned, I can not even search whether it is a good area to call for help or not.

If you are considering option 2, you should be aware whether the station stays open, or has at least a safe place to sit.
That is rather unlikely, but not impossible.
If there is no safe place to sit, you will have to be awake and aware all night and in that case you need to know whehter the area is generally safe.
I would never stay in or near a station through the night if there is an other option, but you might be made of sterner stuff.

As suggested in a comment by @Stephan Branczyk, ask the staff on duty for their name or employee ID, the name and contact details of their supervisor
and if allowed make a video with the staff member telling you they can not do anything more for you.

From the National Rail Conditions of Carriage:

E. YOUR REFUND RIGHTS
26. Refunds on tickets that have not been used
If you decide not to use a ticket (other than a Season Ticket – see Condition 36) to make all
or part of your intended journey, then:
(a)
if the train you intended to use is cancelled, delayed or your reservation will not
be honoured, you decide not to travel and at that time you return the unused
ticket to any ticket office, the Train Company responsible for that ticket office will,
wherever possible, give you an immediate full refund as shown in Condition 27;
(b)
if the train you intend to use is cancelled, delayed or your reservation will not be
honoured, your ticket or relevant portion of it is completely unused, you decide
not to travel and you submit a claim for a refund within 28 days of the expiry of
the ticket to the Ticket Seller you will be given a full refund as shown in Condition
27 as soon as practicable and in any case within one month of your claim being
received.
(c)
if paragraphs (a) and (b) do not apply and the ticket has been bought from a Train
Company’s ticket office (or a self-service machine) and you return your ticket
at any Train Company’s ticket office no later than 28 days after the expiry of the
ticket’s validity, you will receive a refund (subject to the notes below)

So take your ticket to the nearest ticket office and ask for a refund claim form and a complaint form. (I suggest you submit them together, to reduce the likelihood of receiving a small cheque from one section of the company and a meaningless apology from another.) If you also say you are not prepared to accept vouchers for your refund (as is your legal right), it is more likely that your letter will be put to a senior manager who has the power to provide compensation. You need to accept that the only thing this entitles you to is a refund of the fare, calculated pro rata on the part of the journey you could not take; however if you make enough of a fuss the company is likely to do wmoething to keep you quiet.

It is likely that the staff member you spoke to will be the one who suffers for this, despite the fact that he is too junior to be allowed to do anything about the problem. None the less, I encourage you to complain; unless the managers hear about the problems their policies cause, they are likely to continue with their cost-cutting.