DiscountTheatre.com are currently offering 40% off Rigoletto and 29% off Peter Grimes (Dress and Upper Circle only). These aren't huge discounts by ENO standards, but you do at least get to pick your own seat (from a limited allocation). And the postage fee (£2.25) is less than the ENO's own £3.50 charge.
Be warned however that the Peter Grimes seats are nearly all in the back rows. While the website describes them as "perfect if you want a clear view" and "giving you a sense of the bigger picture", they in fact cut off the top part of the stage, meaning there is no view of the surtitles. I also noticed that the Upper Circle prices (original and discounted) had been marked up to Dress Circle levels in some cases, as shown above.
The Rigoletto allocation, which seems to be mostly in the middle of the side blocks, doesn't share these problems.