It would help for us to know what type of travel you have enjoyed previously, and your destinations. Also, are you open to flying to the cruise departure port, or do you prefer to sail from the UK. If you are considering Marella, you might also look at Fred Olsen. Also, how do you feel about cruises with children? Ships can be fuller during school holidays. You might also add Cunard into the mix. And type of cabin. At the time of year you are considering, it might be a bit chilly to get much use out of the balcony many people consider a necessity. For a taster cruise, just to get an idea of what cruising is about, an inside or OV cabin might suffice.
I see you are also not considering P&O, perhaps wisely as they cater more to families. You might drop in on the UK Cruisers board here, a bit further down the main boards page, to see what members think of the UK based cruise lines. I’m sorry I can’t be of more help, but I am strictly US based, where everyone goes to the Caribbean to escape winter. EM