Yes. GB-Sol is the only manufacturer of conventional solar panels in the UK and we have been manufacturing solar panels in Soth Wales since 1999. Romag & Sharp were other notable solar panel factories in the UK, but their manufacturing operations are now sadly closed.
GB-Sol is the only solar panel manufacturer on the Made in Britain website:
As for other companies you might across in a web search for UK solar panel manufacturers, the following call themselves manufacturers but don't claim to have any factories or production assets:
Viridian - yes, they are a British company and they have their own designs, but their products are all sub-contract manufactured in the Far East.
UKSOL - with a name flatteringly like our own, UKSOL was founded in 2015, is privately owned and the 2019 accounts show a net worth of minus £200k. While the website claims "The British Solar PV Module Producer", the small print says that their panels are sub-contract manufactured in the EU, to which UKSOL then add a "30 year British warranty".