How much (£) should i budget for this trip?

"...seriously i'm testing the water, if this trip can be made cheaper
i.e. substitute country A for country B then that's really what i'm
after."

Sure. From London, take Ryanair to Poland, travel around Eastern
Europe by train down to Istanbul, tour Turkey by bus, then fly to
India. Spend time going around India by train, then fly to Bangkok.
Go by road and river through Laos, Vietnam, and Cambodia, then return
to Bangkok. Take a detour to a Thai island beach, and then return to
Bangkok for a flight to Australia. In Australia, you see the country
on an Australian airpass, and if you need cash at that point you work
for a few months and save money for the rest of your trip. Next, you
go to New Zealand, and then fly from Auckland to Santiago, Chile. From
Santiago you travel north through Bolivia and Peru winding up in Lima
where you fly back to London.

That's probably half the price of your initial itinerary, and would let
you experience several regions of the world, and tremendous cultural
and natural diversity.