A useful site listing events all over the country.
Making Music (The National Federation of Music Societies) represents and supports volunteer managed music groups of all kinds throughout the UK. The South East region has 475 groups in Berkshire, Buckinghamshire, Oxfordshire, Hampshire, Surrey, Sussex, and Kent as well as the Isle of Wight and the Channel Islands.
This long established music society organises a series of concerts by both well known and up and coming performers each year between autumn and spring. See the website for details and ticket prices etc.