By car - Plymouth to Santander 18 hours on the boat across the Bay of Biscay
You can also do a channel crossing and drive across France
Pros.
1. No driving (well only about 30km in spain + your distance to Plymouth) This is spot on if you have a very thirsty vehicle (Like me - Landrover).
2. Drink all night in the ships bar.
3. Overnight sailing means you arrive refreshed.
4. You dont have to drive through France.
5. You have your own vehicle in Spain.
6. It is a lot of driving across France + tolls + Channel crossing = lots of time (several days) and high cost. Arrive at Matienzo knackered.
Cons.
1. Ferry is expensive (mind you you still need to pay for a channel crossing of some sort + the cost of fuel through France (about 3 tankfulls each way + tolls)).
2. Limited crossings -Sunday and Wed out, Thurs and Mon back.
3. Bay of Biscay on a bad day
Flying - to Santander or Bilbao
Pros.
1. Quick.
2. Probably more choice on dates etc.
Cons.
1. Need to hire a car in Spain. (I believe you can do it with public transport but I think it involves a degree of walking / hitching, not very convenient with a load of baggege)
2. Small baggage allowance.
3. You are gonna be so surcharged on baggage if you have tent / camping gear, caving gear and diving
If you want to fly it is possible to arrange collection from the airport from people who drove, this avoids a hire car but does limit you in Matienzo.
If you want to sail then you can car share to keep down costs, I know of at least one space in a vehicle going over this easter.
Hope this helps
ps. bring pleanty of beer money
pps matienzo has a mail list, there are links on the website, you can swap lifts, etc there.
www.matienzo.org.uk