I don't live in the UK - but a few shows I am aware of (and there may be more) are:
David Shephard Wildlife Artist of the Year (WAY)
BBC Wildlife Artist of the Year
(this one is free to enter I think)
National Exhibition of Wildlife Art (NEWA)
And In Austraila: Queensland Wildlife Artists Society Inc
Olga it is true you have to have money to make money
This is my almost all artists have a day job for years before doing art full time. There are entry fees, framing costs, shipping charges, etc. that all cost money. I am not aware of any cheap way to exhibit your artwork except perhaps things like local fairs. Have you tried talking to local cafe's and shops to see if they would be willing to display your work for a month or two?