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If I were to show up at the UK border via the crossing from France with my certificate of citizenship (stating my full name & date of birth), and a foreign passport (verifying my name & DOB), would this constitute sufficient evidence?
If you make it plausible that you're an EU citizen, they must let you in. In fact, as soon as you claim EU citizenship, they have to consider your case. That said, you could be delayed while they examine you further.
So yes, your foreign pasport and your citizenship certificate will be enough, as combined, they prove your identity and nationality.
Your passport should not be stamped, but in practice it may happen anyway