Cheddar is bold, salty and creamy. It loves pickles because acid cuts fat and sweetness balances sharpness. For a classic pairing, use sweet brown pickle. For a sharper, mustardy bite, use piccalilli.
If you want a more dinner-like feel, sweet balsamic onions bring a sticky tang that works with mature cheddar. For crunch, add gherkins or pickled cauliflower on the side.
A good cheddar pairing should taste balanced, not sugary. Look for pickles that keep their bite.