England and Wales is a single legal jurisdiction with its own courts and Civil Procedure Rules, and it is the jurisdiction an English law contract needs a service address in. A process agent in England and Wales, at any address within it, satisfies the clause; the wider United Kingdom includes Scotland and Northern Ireland, which are separate jurisdictions.
Why the clause says England and Wales, not the UK
The United Kingdom contains three legal jurisdictions: England and Wales, Scotland, and Northern Ireland, each with its own courts and procedure. Contracts governed by English law give jurisdiction to the courts of England and Wales, so the agent for service must be here, where those courts' documents are served. A Glasgow or Belfast address, however solid, does not answer an England and Wales clause.
Anywhere in the jurisdiction is equally valid
Within England and Wales the law draws no distinction by city: service at a named agent's address in Leeds is exactly as valid as service in the City of London. What matters is that the address is real, permanently staffed and stable for the whole term. Tremark Process Agents operates from company owned offices at Joshua Chambers, 332 York Road, Leeds, LS9 9DN, and has accepted appointments there for clients in more than 60 countries.
If your matter touches Scotland or Northern Ireland
Cross border transactions sometimes involve Scots law documents or Scottish and Northern Irish parties alongside the English law suite. The jurisdictions interact but do not merge, and the right service arrangements depend on your documents; our pages on Scotland and Northern Ireland cover the position, and your solicitors will confirm what each document needs.