You'd think garages like this operating this sort of service all year round would have some form of drying arrangements now, but tbh I don't fully trust em and I'd wait for a nice spell spending 750 sovs.
When I first got mine I looked around a lot for this sortta service and your price doesn't seem too bad compared, but I was never quite happy with the owners assurances and premises, in the end I compromised and addressed the worst areas myself, and then booked it in for a schutz undersealing my Toyota dealer does for 60 quid, but I must have rearranged it 5 times due to the weather that summer until in the end I just said sod it and got it done on a tepid day, but if u don't get it done right can do more harm than good, I think I'd wait for a bit myself personally unless you can drive to Tenerife and get it done there!!
What sortta notice do they give for booking in?