A lot depends on what lavender you have. Musntead and Hidcote are hardy and will survive anything the British winter can throw at them as lo,g as they are well drained and their roots don't get soggy and rot. As mentioned, lavenders in pots may appreciate a layer or two of bubble wrap roun the pot to insulate their roots from cold but the main theing is to make sure the pots are up on feet or bricks to maintain good drianage.
Leave the flowers. They'll be fine. Prune the plants in spring cutting back to the base of the flower stems and a wee bit into the foliage but not as far back as borwn wood as this will not regenerate. Next summer, prune back once flowering finishes and this will stop them becoming leggy and straggly.