I've recently picked up Adam Zamoyski's Rites of Peace because I rather liked his prequel about Napoleon's 1812 invasion of Russia.
Now on page 135, when he talks about Metternich's desire to have a fully-qualified British negotiator at the Allied headquarters for quick decisions of the coalition, he states that:
The British costitution did not permit the head of state to leave the country, so there could be no question of the Prince Regent coming to allied headquarters [...]
This is something I have never heard before and which seems kind of an odd law so I was hoping someone could possibly tell me more about it or at least point me to where such a thing might be written? I'm not really savvy enough with British law to have a lot of ideas on where to look, I tried the Acts of Union without success so far.
From the Care of King during his illness etc. Act, 1811 (51 Geo 3 c 1):
XII. Provided also, and be it enacted, That if His said Royal Highness George Augustus Frederick Prince of Wales shall not continue to be resident in the United Kingdom of Great Britain and Ireland, or shall at any time marry a Papist, then and in either of such cases, all the Powers and Authorities vested in His said Royal Highness by this Act, shall cease and determine.
Other monarchs, previous and subsequent, who left GB to visit territories abroad appointed regency councils to manage things of immediate urgency at home, but George was already acting as regent, so no - he wasn't allowed to leave.