Unfortunately, portable air cons by nature are noisy in general, even the one's that say they are quiet or silent. They're really only silent when you have them running on the plain fan mode, and not the actual air con mode.
Just don't go for an "air cooler" - as these are just glorified fans. You want an actual portable air conditioner.
BTU is the main spec that you want to look for in terms of how effectively it will cool a room. The higher the BTU, the more powerful. And they come as like 6000, 7000, 9000, 10000, 12000.
If you have high ceilings and let's say it's for a standard size living room size, you may want something around 9000 BTU or more. Or for a smaller office room, then a 6000 - 7000 BTU.
For the best price, aircondirect.co.uk have refurbished models that still come with a year warranty.
Other than that, best to shop around the usual places like Argos, B&Q, Homebase, Currys, AO etc.
You're normally better off shopping early, as when the real hot weather comes, they sell out super quick and only the expensive ones are left.