Something worth bearing in mind here is that Austria is considerably smaller than the UK, so it would be more like a South East pass or one just covering Northern's area, or ScotRail/TfW or whatever. The German "BahnCard 100" would be a more viable comparison.
The answer is that a reasonable price for such a thing would almost certainly cost more than I could justify. What I would do, though, is pay around £100-150 per year, ideally by monthly direct debit, for a National Railcard offering 34% discount.
Since the comparison to Austria has been made, I allow myself to weigh in
1. For me, the major advantage of the Climate Ticket (CT) is less the price, but the complete flexibility to use whatever public transport whenever and wherever I want, without taking any additional steps before boarding - buy a ticket (maybe several if I need city transport as well), check whether there are cheap advances available etc. Considering the complexity of the UK ticketing system, that might be one of the major selling point of such a ticket in the UK as well.
2. Arguably, the price in Austria is too low; I would easily fork out € 1500-1800 instead of€ 1000 for the advantages I get, and so would many others. While the ticket per se is an excellent thing, I think that with the low prices, revenue is unnecessarily foregone while I expect no major difference in Modal shift (if you use PT very sparely, even € 1000 is too much; if you use it regularly, € 1500 are easily amortized - my previous season ticket for Vienna and the train to a city 60 km distant was more expensive than that).
3. Even though the UK, population-wise, is approximately 10 times larger than Austria, the price of a British CT would probably not be 10 times the Austrian price, because the average number and length of PT travel will not be 10 times higher. I see the BahnCard100 as a good indicator for pricing too.
4. But I believe there will be two problems
- Number 1 would be easy to solve politically, but probably hard to sell to voters: A CT means lost revenue for transport companies and that will need to be reimbursed; traditionally, while in the UK a lot of tax money goes into PT, railways especially are expected to cover a much larger part of costs from ticket sales, so the amount of state support would be vastly higher than in Austria and Someone would need to pay.
- number 2: train fares historically are much higher in the UK than elsewhere not only because the railways are expected to be self-financing to a larger amount, but for capacity reasons. At least pre-Covid fares in the peak had to be very high to price people off rail because capacity wasn’t there, especially in the South-East and some Intercity routes from London. Now demand might be reduced post-Covid, but there is still the question of whether capacity would cope with strongly increased demand due to lower prices.
If not, there would be a major problem. As can be seen from HS2/NPR, besides its cost, adding new capacity is also a very, very slow process and needs years - whereas the demand induced by a climate ticket will be there almost immediately.
Even here in Austria I am quite concerned that demand on some routes will outstrip capacity - and with a very cheap ticket, there will be less money to add capacity and it would take a very long time.