You might find this hard to believe given the prices, but the margins on rail tickets are too small to ever make this a decent reality.
£4000 of sales would make a ticket retailer £52 in commission.
Once you factor in the overheads of being able to sell rail tickets in the first place, that doesn't leave you much to play with,
especially given Avios are between 1 and 1.5p each.
If a retailer were to offer Avios, the number of avios they could viably offer with ticket purchases would be so marginal it wouldn't be worth the investment in partnering with Avios and altering their retailing systems for the marginal increase in sales they'd see as a result.
The only retailers that can viably do things like this are those such as RailEasy, who can do it because they charge booking fees and credit card fees - and I would advise that the amount you'd pay in these fees is not worth the Avios you'll get from it.
Nectar points are cheap which is why they are an option
Disclaimer/source: I work for a rail company and we tried to offer Avios through ticket purchases.