First things first you need to contact the correct authorities. You require written permission to trap crayfish in the UK. To find out where you can trap you fill out the application form (Gov doesn't publicly list it). There was an episode of Rover Cottage where they trapped them on the River Kennet.

But you will also need landowners or angling club permission! 

> If you trap crayfish without written permission you could be prosecuted.

Bonus however of the fact that the crayfish licence is free currently. 

For the traps you must follow the guidelines out by Gov.

> **Crayfish trap rules**

>There are strict rules about the design and size of crayfish traps because they can harm other wildlife. If traps are the wrong size or design your application will be refused.

>Traps must:

>- be no longer than 600mm
>- be no wider than 350mm at the widest point
>- have an entrance no more than 95mm wide
>- have mesh no bigger than 30mm at its widest point
>- have EA identity tags

>You must:

>- tell EA or NRW about crayfish you trap using a catch return form (you get the form when you’re authorised)
>- return species not covered by consent to the water they came from

As for the native species, these can be trapped soley for scientific research, and you again require written authorisation to do so.

**Be aware** when catching a invasive species you cannot release it back where you find it.

[Source: Gov.uk trapping permission pages][1]


  [1]: https://www.gov.uk/permission-to-trap-crayfish-eels-elvers-salmon-and-sea-trout#crayfish-trap-authorisation