There are many local botanical art groups in the UK either open to everyone or with some selection criteria - some just meet to paint, others have a programme of invited speakers/teachers/demonstrators.
Amicus Botanicus
Bath Society of Botanical Artists
Birmingham Society of Botanical Artists
Cambrian Botanical Artists
The Eden Project Florilegium Society
Royal Botanic Gardens Edinburgh Botanical Art Society
Gloucestershire Society of Botanical Illustration
The Hampton Court Palace Florilegium Society
Highgrove Florilegium
Horncastle Botanical Painters
Independent Botanical Illustration Society (Rotheram)
Institute for Analytical Plant Illustration (Birmingham)
Leicester Floral Artists Society
Leicestershire Society of Botanical Illustrators
Mercia Botanical Artists Society
Botanical Artists of Ness Gardens
The Northamptonshire Botanical Artists Society
The Northern Society for Botanical Art
Nymans Garden Florilegium Society
The Florilegium Society at Sheffield Botanical Gardens
The South West Society of Botanical Artists
Botanical Art Society of Sussex
The Worcestershire Society of Botanical Artists
It is possible to get grants for illustrating particular projects. You do need to fulfil certain criteria, but it can be a good way to pay for the costs of hanging an exhibition, raising awareness of your group, and getting some extra tuition and advice. One group has received £3950 from the lottery for a Study of the Vascular Plants of Anston Stones Woods.