>>1396890when starting off a second hand pos will do you just fine, you wont have the skillz to actually make use of even a £500 bike.
go on ebay or something or go to a recycling centre, thats where i got my mountain bike and its been pretty good, aluminium ovular tube frame will proper rear suspension.
when you start getting good enough that the bike is holding yo back you can either buy a proper one or just upgrade the cheap one