More information needed - what dates? Miami-UK cruises are usually the repo cruises at the start of the summer season which is around April. TransAtlantics are usually very reasonably priced too.
If you check out ‘discover cruises uk’ this shows you all the possible transatlantic options. There are a number of transatlantic crossings and they seem to go from Miami, via the Canaries, to the UK. I don’t know how long your holiday would be but a problem may be the length of time involved in the sailings from there to UK. Just go to the discover cruises website, go to ‘cruise finder’ and then ‘transatlantic crossings’. Hope this helps.