Can't recommend a company but I can give you a newbie heads-up. I was given
a second-hand green house. I laid a concrete plinth and built it upon this
secure in the knowledge that the greenhouse was very heavy and would be
there for years to come.

WRONG! The wind blew it about 6 inches across the garden. Fortunately the
mastic & glass meant is was rigid enough to just slide without breaking any
panes but getting it slid back was "fun".

Now it's bolted to the concrete!

BTW, glass is much stronger than you might expect. I have a 14 month and a
3 year old and apart from the impending attack of footballs and the like
(odds are my "return kick" will be the first one to break a pane!), I don't
worry about the glass.

Paul DS.