Hi James,
I did a few long haul rides without panniers last year and used a 'seat pack' which is part of the bike packing (as mentioned by Dave) luggage now used as alternatives to panniers.
I used this one if I was riding through the night:-
http://www.wiggle.co.uk/altura-vortex-seatpack/
So I could carry a change of lycra etc, but also it was one of the cheaper options. The swanky ones are made by Apidura (UK company) lovely but they are a fit bit more expensive. Again as mentioned Ortleib also do an option but also a bit more expensive.
Have a look for handlebar pack and/or frame bag as an addition or even alternative. The frame pack gives you a bit more balance to the bike and allows for a little 'easy access' saddle bag instead of the big boy.
3 of us did Paris to London in a day last year which was quite an experience.
We got the Eurostar form London to Paris with the bikes all packed in boxes (they only allow 2 bikes per train so we had to beat the system and treat it as luggage) wearing old clothes. When we arrived in Paris, we built the bikes on the platform, got changed into our kit and threw the old clobber in the bin.
Cycled all day to the Ferry and then through the night back to London. We only took gilets and arm warmers with minimal food supplies (plan your route around civilisation for food stops - service stations are the most reliable) and split our tools out between the 3 of us to keep weight down. Couple of multi tools, number of inner tubes, 1 pump, CO2 inflator etc.
I didn't use the bike pack for that trip, so if you are, aim to rinse your gear on the night and you'd be able to fit some light clothing and some flip flops in there fairly comfortably - light running style T-shirt, lightweight walking trousers both pack down small.
If there's a bit of a chill on the evenings, re-use your gilet and arm warmers. It'll make a real difference to get out of your sweaty clobber of an evening and put on some 'civies'.
One thing that makes every morning a joy to start - tooth brush - it'll brighten your day I tell thee.