The bigger difficulty is that they might have had to bring up the copper in order to be able to distinguish copper from silver. I'm sure this was the reason why they stopped last year even though they were finding bars.
This year they went back and determined exactly where the silver is stored so that they won't have to deal with the copper. Even at the shallower depth of the Mantola, bringing up the silver from the bottom is going to be the time bottleneck, so getting just silver and not copper is important.
To get some idea of the difference, if they were to bring up as much copper as the amount of silver they found on the last load from the G, even if the copper were pure (which it may not be), it would only be worth about 7000 dollars per ton, so they would have enough to pay for about 4 days of the SBW. If it took more than 4 days to bring up 60 tons of silver from the G, bringing up copper from the M would probably cost more than it is worth.
They would only bring up copper if it is interspersed with the silver and unable to be distinguished, and they can still bring up all the silver in a short enough period of time to cover the costs of the SBW.
The UK does pay the expenses in the form of reimbursement from proceeds recovered in the Mantola salvage. It is risk less to the UK if no recovery is made, but OMEX receives first dollars out as reimbursement for expenses.
The Victory will work differently in that OMEX will receive their expenses regardless of the amount of recovery. The mechanics of this was one of the pieces holding up formal approval.