Its about 400km on the other side, about 4/5 hours from Calais - the autroute is quiker but the Route Nationale is more fun. Portsmouth may well be closer but its another 2 hours from Dover for me ......
Chunnel can be had for £49 each way if youve got the skills, Ferry is anything from £25 upwards.
Youve got roughly a month to decide, apparently its selling out quick so I need to let the man know around Christmas time, get involved