don't know if you have a satnav or not,, if not get a decent map of both France and Spain as you will be wanting to use the smaller roads, not the main arteriel roads.

To save money.
1, get a cheaper ferry
2, avoid peages wherever sensible.
3, camp free.

How to do that

1, take the LD/transmanche ferry from newhaven/portsmouth. To get there come down the A34 as far as possible and if sailing from nehaven use the A272 ( the road that goes through Bilingshurst).

2, From the French side head for Rouen, Then take the peage A13 to Evreux via Louviers, USE THE PEAGE, it will cost 1 euro but save a lot of hassle.
Stay on the N154 until you join with the RN12.
Then turn right to head west to Mortagne au perche. head for Belleme then Bonnetable, as you approach Le Mans turn onto the D20 BIS until it joins the D323 (old RN23). Turn off at Arnage and follow the D307 to Samur.
At Samur follow the signs for Niort until you get close to Parthenay after which follow the signs around the ringroad to Poitiers. Look out for St Maixent l'ecole. follow the signs for that then on to La Mothe st Heray and onto Lezay and Sauze Vasous. Here you pick up teh RN10 south to Bordeaux. Follow the ring road around towards Bayonne but turn off at J16 onto the d1010 which is the old RN10 follow it all the way to St Geours de Maremme which is where it crosses the RN124.
From there follow the D12 to Urt, Hasperran and Cambo les Bains. From Cambo follow the D918 to Ainhoa south and onto Sare which is close to the Spanish border. Follow the D406 and it will take you to Bera de vidaso in Spain.

3, The Aires/laybyes are meant for people to stop, rest and sleep. Generally an overnight 'Bivouac' in little tents with your bikes parked close by will be fine. Often they have showers as well as toilets although basic. It helps if you buy a coffee in the shop if there is one. Generally the busy Aires are safer as there is lorry traffic about. Pitch up out of sight of the road if quiet site.
Look out for Supermarkets to buy food and fuel.

but do get a decent map, ideally one that is ring bound as they open up better.