Yeah, i dont like that practice either. They did it with Elite: Dangerous, and it stopped me from buying it on a whim as a result, and once i'd watched more videos i quickly realised it probably wasnt going to be what i wanted (a stop-gap till Star Citizen had more game content) as it seemed a little hollow and repetitive.
IMO, paying the RRP is more than enough. You're getting access to the game before its finished, and any problems you have you're providing them feedback, you're an audience to get feedback from etc, and the reward is getting it 'as-is' before its finished.
Ignoring the fact that RRP applies to only Origin, Steam etc for illegal price-fixing reasons to keep retailers happy and mistakenly feeling relevant, you can get most games for about 25% cheaper ahead of release & shortly after, but then to charge a premium to be the tester too, thats taking the mickey. Adding 66% onto the RRP is an insult.
Its also worth pointing out that the £29.99 is also a limited time price. They *could* put the price up to £34.99 tomorrow, or £49.99, we dont know. Maybe they'll wait till a month before release, maybe they'll give advanced notice '1 week left at the discounted price, pre-order now for big saving' etc.
Going back to the beta-access price, I can only assume they do this because A) they can, and people still pay. B) They probably dont need that income as they have a publisher financing them. C) Too many people, too many voices. You have say 5000 people who're so interested in playing their game they're paying extra, are likely to be a good sample of their core audience with a like-minded view, and not people that picked the game up with a casual interest, muddying the waters.
-edit-
I think Elite was £50 for beta, and £40 for release pre-order. Its possible this is following the same path, if it was originally £30 and later went up to the £40 price.
Looking on kickstarter, digital copies went from £20 (first 10k), £25 (5k), and £30. Then at £50 is "Participate in the second round private beta test, plus all rewards above.". So its following the same route, so maybe theres info out there as to when they bumped the game price up to the £40 price. Whether it was 6mo before release, the first month, or when they released the launch date.