Should i pre book crossing Dover to Calais or will i be ok just turning up in August ?

[fakeblondie]

Im looking at the cheapest way to cross tbh, however if I pre book tickets we risk losing our paid for ticket as we are driving to South of france and could end up running late.
If I pay for flexi ticket on ferry or Eurotunnel its around 400 return.
Id like to use my Tesco clubcard vouchers if possible but cant use these for a flexi ticket.If we get stuck in traffic ect we might miss the Ferry/tunnel.
Cant use Tesco for ferry, so :
If I book 1 way set time on Tunnel with my vouchers and turn up at the Ferry (kids would love to do both anyway ) on return a) will the price hike up nearer time and be much more than its shows now and b) bank hol weekend in Calais in August will we actually get on ?
TIA

From experience I would book now, and get there in good time...that will be the cheapest way to do it.

I know there is the concern about timings when driving from south - we have this every year. We tend to stay somewhere round Chalons en Champagne so we know the drive is about 3 hours.

Book now.

Arrive early and have breakfast /snacks. I've been let on the earlier ferry a couple of times if you're there in good time but that will depend how full, and in august its likely to be full!

Thanks both I`ll book now x