I am currently riding freestyle on a cheap unicycle but I really want to be able to do street/trials riding and I was wondering whether to get a Koxx one or a Nimbus. I am able to get something up to around £270. 
get the K1. that’s what i’m doing in two days. the reason i say this is because the K1 is normally (at least the alien backflip is) around 840 USD, (600 euros), but they are being liquidated for the price you mentioned. i have seen a K1 take insane abuse, so unless you just hate the way the K1’s look, i would jump on that deal before it’s too late.
HOWEVER, i am no uni authority, i’m looking at this in terms of economic value vs. quality. trying to be as cost effective as possible. If someone more educated says otherwise, i would listen more to them 
If you look around here, most people are riding KH or K1. Nimbus is really a budget option.
K1 has been going dirt cheap for some models on a few websites, if you can still find them at the price then I’d go with one of those. Not sure if those deals are available in your area because you’re stating pounds and the sites were American I believe.
When I got my older model K1 it was cheap at £210 - pretty sure the K1 unicycles are only cheap in the US atm - aint seen any cheaper in the UK yet.
I just got a k1 alien backflip for $240 USD. really good deal. haven’t been able to try everything on it yet but its amazing for flatlanding. I’ve also heard it can take up to around a 5-6 foot drop so I’ll be gradually testing that out.
extremely sturdy unicycle though. definitely get one of these k1’s if you need a solid trials/street/flatland uni
I just got a k1 alien backflip for $240 USD. really good deal. haven’t been able to try everything on it yet but its amazing for flatlanding. I’ve also heard it can take up to around a 5-6 foot drop so I’ll be gradually testing that out.
extremely sturdy unicycle though. definitely get one of these k1’s if you need a solid trials/street/flatland uni
I got a K1 black domina II during the sale for $180 total after a coupon and all. I absolutely love it. This thing is built to take some damage. I’m not an extreme rider yet, but I’ve done 3ft+ drops on it and it doesn’t even make a sound. I’m very glad I hopped on the K1 deals. There’s lots of people who ride nimbus though, and they seem to really like em. I don’t think you’ll be disappointed either way.
They have been going CRAZY cheap, shame I can’t get one here in the UK. I’ll have a look at international shipping actually.
Wow! Thanks everyone, as the answers are so unanimous I will go for a K1:) Does anyone know if the white russian is any good for street/flatland, I have found it for £265 + p&p on unicycle.com but I will have to wait for them to re-stock as it is currently out of stock.
I’d personally say that you want a unicycle with a flat crown if you get into flatland.
If something’s out of stock on the UDC UK website - normally best to send them an email - then you can find out when it might be in stock and they are normally really helpful - might find they have one hiding in a box somewhere - you never know
What is a flatcrown? Doesn’t the white russian have one.
The Koxx White Russian Frame (Devil frame) is like this:
A flat crown frame is like this (the crown is the horizontal area just above the wheel):
I have a devil frame on my white widow but I’m not good enough to try and use it for anything yet so I have no issues with it - I’m sure many people ride without a flat crown but I think the people who have flat crown wouldn’t go back 
OK I think i will get a white russian and then when I need it I will change the frame to an alien backflip, if I can.
well if you’re going to change to the alien backflip/black domina/green spirit frame you might as well buy it now. You won’t be able to find it for just an additional $20
Are the black domina, green spirit and alien backflip the same except for the colour?
Is the alien backflip suitable for street/trials as well as flatland?
Yeah I believe the green spirit, alien backflip, and black domina are the same, EXCEPT, the alien backflip comes with cheaper pedals. The butterfly pedals that come on the black domina and green spirit are like, super trials/muni pedals. REALLY grippy, and expensive. So most bang for your buck would be a green spirit or a black domina. But if you don’t want to use those super grippy (and dangerous) pedals, then you’re gonna need to buy some $15 pc pedals or something.