Better to use a UK-specific source for this information. The "horse's mouth" is the Ofcom transmission parameters spreadsheet, which is the most up-to-date, and I think this is where Michi's data comes from:
Details of the technical parameters of all analogue VHF, MF, and DAB transmitters (including services on multiplexes) currently on-air.
www.ofcom.org.uk
MB21 has more information, including usually pictures of the transmitters in question and some interesting technical and historical stuff. You can choose a station to see all its transmitters on a map:
The sole remaining BBC Radio Wales AM transmitter is at Washford, in south-west England, with a clear sea path across to south Wales: