Finding Flint to Knap is one of the biggest problems a UK knapper faces, here I hope to make the task easier.
Most professional and serious amateur knappers get their flint from chalk quarries where it is a valuable waste product sold on to the building industry and some of these quarries will happily supply flint to the visiting knapper. However the quality of flint is not the same at all quarries, grey flint mottled with inclusions is largely useless for knapping, where as pure black flint priceless, most knappers use flint somewhere in-between and enquiries will have to be made to individual quarries.
An updated list of every chalk and chert quarry in the UK is found here as a downloadable PDF
https://www2.bgs.ac.uk/mineralsUK/mines/dmq.html
Also a list of quarries people have visited around the country with reviews is found here.
https://uk-knapping.forumotion.co.uk/f3-resources
Another source of flint is beaches. Much of Britain has white cliff beaches, in amongst the chalk cliffs bands of flint nodules can be found and often beside the cliffs fallen nodules from sea erosion.
A couple of famous beaches for this are Beer Beach in Devon and Cromer Beach in Norfolk. Some Thames beaches in London are scattered with flint used to make gunflints in the age of sale. This may be same in other parts of the country age of sale warships were stationed.
Many Chalk Hills will contain flint, I've found eroded nodules at the bottom of hills in Dartford, Croydon Somerset and Suffolk.
Old Chalk quarries are often turned into housing estates, shopping centres ansd so on and still have the chalk side visible, founding out about old chalk quarries in your area is always useful.
Another source is building merchants, many around the country sell bags of flint for building purposes.
Finally many US based knapping companies sell Chert, Obsidian, Jasper Dacite, Agate and so on, often sawn into slabs for convenient posting.
Another alternative is glass. Glass bottle bottom knapping isd a common beginners activity, however a superior alternative to this is magnifying glass lens knapping which can be bought cheaply from pound stores and 8mm glass tiles purchaseable from eBay.