Ultra Wide Angle for K1

I need some advices, I want to buy one, but not spend more than 400 pounds. Please advise. Second hand and old ones are OK too. I don't need zoom I don't need auto focus.

What is your definition of "ultra-wide" ? 24mm, 20mm, or wider?

Second Hand you could look out for the Sigma 20mm f1.8 - but they aren't too easy to find now FF is around. Pentax have the FA and A 20mm f2.8 - both usable if stopped down sufficiently. The Pentax FA 20-35 is on of the best all-rounders, but again difficult to find.

At 24mm there's a bit more choice, and the quality tends to be better (at the extreme edges in particular).

The Samyang 14mm f/2.8 is a decent ultra wide angle.
https://www.pentaxforums.com/userreviews/samyang-14mm-f2-8-ed-if-umc.html
Manual focussing though (but the depth of field is huge.)

As you don't mind manual focus I also think the Samyang 14mm would be a good choice. Personally I prefer auto focus so I went for the Sigma 12-24. A big slow lump but it does the job on the K1.

isn't the samyang the one with the distortion too hard to fix? Because otherwise it would have been my first choice too. The Sigma 12-24 was also my favorite some time ago, until I read that it's not that sharp.

for UWA I mean less or equal 20mm

It isn't but if you want a F/F A/F wide zoom there is not much else except the much more expensive new Pentax 15-30. Another option is to put your K1 into crop mode - then you have a much wider range of lenses to choose from.

Samyang 14mm photo just taken for you

(Note: The exif shows 15mm as the K-1 doesn't offer 14mm as a manual lens focal length setting.)
What distortion correction does that need? (I think you're thinking of the 12mm fish-eye, but even that can be corrected with hemi-fisheye.)
The Sigma 12-24mm isn't particularly sharp. OK for web images but not a patch on the resolution of the Samyang.

I personally have the Irix 15mm Firefly and love it. The build quality, the handling, the focus lock (such a brilliant idea!). At this focal length, you practically do not need to touch the focus. Mine is set and locked on the spot on infinity mark and most times I forget the focus ring is even there; unless I am trying to be weirdly creative

This lens is a little marvel imo. The colors are fantastic, distortion nearly non existent. The only 'issue' it has, is it flares massively if you have the source of light directly in the centre of your frame. The moment you shift slightly off centre it's manageable as you can see on the photos and the star burst it produces is lovely!

Ah, the little baggy/pouch it comes with is not very convinient to use and only offers dust protection. I actually purchased the hard shell case for it which is great.

Some pics:

the 14 mm has the so called "mustache" distortion that is almost impossible to correct, but I don't know how serious it is. About the Irix I also read not so flattering reviews :/..but yeah those were the lens I had in mind as well, I have been stopped by these apparent problems.

I may go for the Irix tho

I doubt you'll find any ultra wide angle without any distortion. By definition it's inevitable as it's outside the normal field of view. In real life photos though (as opposed to a test charts) I doubt you'll be aware of anything amiss.
To top it all, you don't want to pay more than £400 so the Samyang fits within that easily, I believe the Irix would double that. If you paid £4000 I doubt it would be free of distortion which makes the Samyang a true bargain for what it offers. As I asked, what distortion needs correcting in that image I posted?
Manual focussing of an UWA is not a problem given the huge depth of field, indeed AF is overkill.
Another option might be to explore the Pentax UWA's, eg the K or A 15mm f3.5, or the K 18mm f3.5 which might just come in at about your budget.

Irix costs just over 400 quid for the Firefly. Distortion wise, it performs way better then the DFA 15-30mm at 15mm and I believe better then the Samyang as well. You can quite easily look through reviews for the numbers if it's that important to you. Not quite sure where you read negative reviews of it?
Either way, both lenses are fantastic value for money. Easiest thing is maybe to order one, have a play and decide if it's worth keeping or find a store that will let you fire some test shots?

The Sigma 10-20 or Pentax 12-24 will work. Got first hand experience of the 12-24. It covers the full frame from about 18mm. From what I've read the 10-20 covers it from about 15mm. As incidentally does the 10-17 fisheye.

That's interesting. I'd never thought to try my Tamron 10-24 on the K1 until I read your post. I just tried it and it also seems to cover from about 15mm. I must test it more thoroughly tomorrow.