tl;dr: once it's booked, they should honour it or compensate (except in the case of not-at-fault delays on a minority of operators)
This is for the Eurostar, but I guess the question is general for UK trains
Nope. In fact, many of the franchised national rail operators are different to each other, let alone Eurostar.
Eurostar
Eurostar's delay compensation policy includes the following:
If you haven’t travelled yet and your train is cancelled or likely to be delayed by more than an hour
You may decide not to travel, in which case you have 60 days after your original travel date to:
- Exchange your ticket for free. You can travel anytime within 180 days of your exchange request (90 days if your booking includes onward travel with another train company), subject to availability.
Or
- Request a full refund, regardless of your original ticket conditions.
Clearly, then any ticket rescheduling after you have booked constitutes a delay; thus, if you do travel, you will be entitled to the regular compensation. The delay quoted in the automated email may well be as innocent as the rescheduling falling through the cracks in their computer system - if it makes a material difference to a claim, I would recommend speaking to them in person/on the phone/via email.
Franchised Operators
This includes most (but not all) domestic rail services in Great Britain. The compensation specifics vary between train operators, but (apart from the older franchises, like GWR) most are now on some variant of the "delay repay" scheme. On this scheme, you are entitled to a sliding scale of compensation, increasing as the delay increases, whatever the cause of the delay. For some operators, this kicks on on delays as short as 15 minutes.
Some franchises are not yet on this scheme, and have less generous compensation arrangements. GWR, for instance, do not compensate for delays "outside [their] control". They also have different arrangements for season ticket holder than the delay-repay operators. For the rest of this discussion, uncited statements come from GWR's passenger charter; it should be representative of the least generous operators.
Advance tickets can be purchased up to 12 weeks in advance; planned engineering works are booked at least 12 weeks in advance. Thus, they will be known about when you book your ticket, so no schedule change required. (I can see an edge case, if you can buy a 30 day open return for outward travel in 12 weeks time - I don't know if you can do that, but I think you'd take the risk of engineering works in that case; this is also the case for long-duration season tickets.) Emergency/short notice engineering works count as a delay, the same as any other. Delay-repay operators should repay, for others it depends (e.g. if the works were to repair vandalism, you wouldn't be covered, but if to fix faulty points, you would be). In addition, you may be entitled to cancel your travel plans for a full refund, even where the ticket doesn't usually allow this.
National Rail Enquiries has links to the compensation rights and passengers' charters for all train operators.
Open Access Operators
A small minority of services on the national rail network are operated by open access operators - these are train companies that pay for access to track/facilities, rather than operating under contract to DfT. These include Grand Central, Hull Trains and Heathrow Express. In theory, these are a law unto themselves, but in practice I think they're comparable to their franchised brethren. Check the relevant operators websites for details.