This morning I have a (partial) answer.
Part of the cost structure is billed by giffgaff, (data +calls+send text), and part is in the hands of the non terrestrial service provider, (the receive texts/MMS ).
The charges in www.giffgaff.com./help/articles/usingyourmobileabroad quotes charges for non terrestrial systems as the "rest of the world" rates,
£1/min make and receive calls,
30p to send text.
These are correct according to a giffgaff agent. This is what you should be charged for those services while at sea.
Charges for send/receive MMS and receiving texts are not in giffgaff control (because no arrangement with Telenor?) and therefore they say you have to contact the maritime service supplier Telenor Maritime in my case. I have no cogent response yet from them.
Other operators (3, EE) show a tabulation for non terrestrial/maritime connections in the country list of their international pages. Notably several suppliers have deals in place that allows the supplier to state transparently the full range of charges faced when at sea.
I have suggested giffgaff do the same, quoting the charges they have control of, and put in "contact supplier" or some such for receive text and MMS. As it is now it is very hard to find a clear answer.