Gov.uk has a detailed guide outlining how to cross the UK border with cash. Here are some of the basics to know:
You must declare cash of £10,000 or more to UK customs if you’re carrying it between Great Britain (England, Scotland and Wales) and a country outside the UK.
If you’re travelling as a family or group with more than £10,000 in total (even if individuals are carrying less than that) you still need to make a declaration.
If you’re travelling to Northern Ireland and carrying more than €10,000 (or equivalent) you must follow the rules for taking cash into Northern Ireland.
The earliest you can make a declaration is 72 hours before you plan to travel.
You must declare cash of €10,000 or more if you carry it between Northern Ireland and any non-EU country. You must also declare cash of €10,000 or more if you’re arriving in Northern Ireland from Great Britain.
You do not need to declare any amount of cash when travelling the other way, from Northern Ireland to Great Britain. If you’re travelling as a family or group with more than €10,000 in total (even if individuals are carrying less than that) you still need to make a declaration.
Customs authorities may ask you to fill in a cash disclosure form if you send cash by freight, post or parcel between Northern Ireland and any non-EU country. This includes if you send cash from Great Britain to Northern Ireland.