Where does the Data Protection Representative have to be located ?

Your EU GDPR representative must be located in a (single) country in the EU where the individuals whose data are being processed are located. If your company generally targets the entire EU, then it can choose the country where it wants to base its representative. As Brussels is the capital of the EU, it is a preferred location for non-EU companies to designate their GDPR representatives.

Your UK GDPR Representative must be located in the UK.