I’ve booked Eurotunnel for the car in a week’s time. They are being quite flexible about the tickets, if you need to make changes, the ticket it will be valid for a year I think.
I’m prepared for changes, Spain might enforce quarantine for UK travellers in a tit-for-tat exchange and who knows, others might follow. If that happens, I won’t go.
I’m also prepared for a quarantine period on return, if necessary.
I have no idea if the tunnel is taking bikes - you will need to check - for car traffic they are insisting you stay inside your vehicle for the entire crossing, WCs in the carriages are not available.
I don’t know about Belgium but the Netherlands has remained mostly open for business even during the worst of lockdowns elsewhere so you might find it pretty much unchanged, indeed it might be the perfect destination at the moment.