There’s nothing more (or less) “official” about any deed poll than any other: don’t be fooled by those who’ll try to charge you!
Yes; it really is this easy. Legally-speaking, there’s no reason that you HAVE to have a witness at all. However, it dramatically increases the likelyhood that your deed will be accepted. If you do have witnesses (and you should!), then you should theoretically pick people who are reputable (which used to mean being male, over 21, owning property, etc., but now basically means being anybody who can be identified should the need arise) and unbiased (which means that you shouldn’t use a spouse or family member, or anybody who could conceivably have coerced you into signing).
If you look around the other “paid” deed poll sites (and there are lots of them, charging anywhere between £10 and £100), you’ll note that they all have different rules about witnesses!
When I changed my name, I took two of my friends down to the pub and bought each of them a pint in exchange for some signatures. When my partner changed her name, she used me and our lodger. Most of the folks I know (many of them from their feedback on my website) simply coralled any two friends and used them. And that’s all you need to do.