The longest words in Oxford Dictionaries Online are:
antidisestablishmentarianism - opposition to the disestablishment of the Church of England - 28 letters floccinaucinihilipilification - the estimation of something as worthless - 29 letters pneumonoultramicroscopicsilicovolcanoconiosis - a supposed lung disease - 45 letters
You're unlikely to come across these words in genuine use: they're generally just provided as answers to questions about the longest words in the English language. In terms of sheer size, however, the longest word to be found in Britain is the Welsh place name Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch. For obvious reasons, it's usually abbreviated to Llanfair PG.
The 20-volume historical Oxford English Dictionary includes other very long words, most of which are highly technical. These include:
otorhinolaryngological - 22 letters immunoelectrophoretically - 25 letters psychophysicotherapeutics - 25 letters thyroparathyroidectomized - 25 letters pneumoencephalographically - 26 letters radioimmunoelectrophoresis - 26 letters psychoneuroendocrinological - 27 letters hepaticocholangiogastrostomy - 28 letters spectrophotofluorometrically - 28 letters pseudopseudohypoparathyroidism - 30 letters
People sometimes ask whether a DNA string can be considered as the longest English word, given that they can run to many thousands of letters. The answer is no: they're regarded as chemical names rather than genuine words in the sense of