Yes, but that is really rarely used -at least around here-. Most envelopes nowadays have opted for different factors. Usually you fold an A4 in thirds on the longer side and use an 110x220 envelope.
Folding in thirds isn't worth the bother when I post things so infrequently.
Choosing a large British supermarket, they have C4, C5 and C6 sizes, plus DL (A4 in thirds) which are cheapest: https://www.tesco.com/groceries/en-GB/search?query=envelopes