Is a nikah valid without a wali (guardian) in the UK?

It depends on the madhab. According to the Hanafi school (followed by most UK Muslims), a nikah without a wali is valid if the woman is a sane adult and the husband is a suitable match (kafā'a). If kafā'a is absent, the wali may annul it. According to the Maliki, Shafi'i, and Hanbali schools, a wali is a condition of validity; a nikah without one is invalid. It is strongly recommended to involve the wali to avoid doubt and family conflict. Note: Regardless of Islamic validity, a nikah-only marriage has no legal standing in UK civil law; a separate civil registration is required for legal protection.