Brittany Ferries - cheap but useless dates?
I just checked the Brittany Ferries website and the only outbound dates which would be of use for this event Santander route are Tue 14th and Sun 19th July, so assuming it's still a 24 hour voyage you'd either be there nearly a week early or miss the prep day. Even better, in terms of return dates they offer the equally useless Wed 22nd or Mon 27th, so if you wanted to make use of the planning day and enjoy the full 2 days of HUMM you'd be looking at near enough a 2 week expedition. Which is probably enough time to walk to Spain.
P&O's Bilbao route is marginally more convenient, so assuming the same requirement of wanting to attend the full 3 day event you'd be looking at leaving on Thu 16th (arriving early 18th) and heading back on Fri 24th to arrive the next day. I know it's nearly 50% more than the Brittany offering and that the outbound leg boasts an additional 12 hours to spend in the company of Tattoed Ned and his Bearded Wife, but factor in all the extra accommodation you'd need and Brittany doesn't seem that cheap after all.
More malformed, irrelevant opinions here.