I don't know any of these personally but
http://www.ukfolkfestivals.co.uk/festivals-this-month.php
gives pretty comprehensive cover of all folk festivals :

Mull of Kintyre Music Festival - 21st Aug 2019 - Campbeltown, Argyll and Bute

Cornwall Folk Festival - 22nd Aug 2019 - Wadebridge, Cornwall

Dunmore East Bluegrass Festival - 22nd Aug 2019 - Dunmore East, County Watford

Shambala Festival - 22nd Aug 2019 - Secret, Northamptonshire

Cottingham Folk Festival - 23rd Aug 2019 - Cottingham, Yorkshire

Greenbelt - 23rd Aug 2019 - Boughton Estate, Kettering,NN14 1BJ, Northamptonshire

Marymass Folk Festival - 23rd Aug 2019 - Irvine, Ayrshire

Maui Waui - 23rd Aug 2019 - Theberton, Suffolk

Shrewsbury Folk Festival - 23rd Aug 2019 - Shrewsbury, Shropshire

Solfest - 23rd Aug 2019 - Tarnside Farm, Tarns,CA7 4NQ, Cumbria

Stroud Fringe - 23rd Aug 2019 - Stroud, Gloucestershire

Towersey Festival - 23rd Aug 2019 - Thame, Oxfordshire

Watchet LIVE Music Festival - 23rd Aug 2019 - Watchet, Somerset

England's Medieval Festival - 24th Aug 2019 - Herstmonceux Castle, BN27 1RN, East Sussex