Counting Child records on Parent record form

Hi

Is there a way to display in a number field the 'Count' of contacts on the Accounts form?

I would require a field on the Accounts form called 'Contacts UK Count' for example that would look at the contacts of that Account and if the Country = UK then it would add/subtract on that field every time the form is opened.

Is it possible through calculated fields, business rules or would it have to be a plugin?

Thanks