possibly Head Gasket Failure:

Been told by my mehchanic the HG has gone on my uk 2000 scooby
was over heating the heaters are blowing cold until i rev it up past 4500rpm, also theres water being chucked from exhaust pipe
and the radiator bottle water rises to the top and chucks water all out

funny thing is when the AA guy turned up to tow it he checked it and said it was the water pump as some pipes were warm and some cold?
but he didnt carry out the compression test.
also he said the waterpump was a sign when i rev it they blow warm air through heaters,

is it possible for head gasket to go like this?

 

No hot air from the heaters and coolant overflowing out the expansion bottle is a sure sign the HG has gone. Sniff test/compression test will confirm.

 

my mechanic said the same and also checked exhaust and its chucking out water more then normal,
we bypassed the thermostat and still its doing same thing, we put thermostat in hot water and it worked fine.

 

Your "mechanic" has done all that, yet not done a simple "sniff test"?

Get a decent mechanic to do one for you.