Sources of used trumpets and cornets in the UK

If you are looking for a second-hand cornet or trumpet in the UK you have a number of options.

Online Instrument Dealers

Most specialist brass shops have websites and will sell by mail order. Here are a selection of links to the second hand pages of some of these dealers:

John Packer
Hayes Music
Prozone Music
Band Supplies
Phil Parker
Dawkes Music
Duchy Brass
First Brass
Trevor Jones
The Wind Section
Trevada Music (trumpets)
Trevada Music (cornets)
Once Loved Musical Items
Reeves Brass
Gerry Birch Music Services
Bexley Brass
Musical Money
Windblowers
Headwind Music
Musical Money

Non-Specialist Second Hand Dealers

Cash Converters
Cash  Generator

Ebay

Here are links to searches restricted to results from the UK:

Trumpet
Cornet 

Classified Ad Sites

Gumtree, will return national listings so you will need to narrow it down to your area.
cornets for sale on Gumtree
trumpets for sale on Gumtree

Physical Auctions

You need to pay buyers premium and they won't usually post items, you need to be able to collect. There are a few online bidding services used by auction houses. Here are some of them.

the-saleroom.com
ukauctioneers.com

Charity shops and pawnbrokers

You might find trumpets and cornets for sale in charity shops and local pawnbrokers. This would require regular visits as their stock changes regularly.