This list is in the order of popularity, organized by the search ratings in 2021 via Google.
Please note while these are all UK based online retailers because of Brexit some are unable to deliver to Northern Ireland, so please check first. There are many other excellant online retailers in addition to this search engine result list.
Thompson and Morgan Legendary seed young plant and huge range of plants including new novelties.
Primrose.co.uk Wide range of plants and garden products plus Primrose TV.
Prestigeflowers.co.uk Massive range of flowers and partnered gifts.
Bunches.co.uk Cut flowers, Flowers by post, Hand tied bouquets, Birthday Flowers and gifts etc.
BloomandWild Through a thoughtful Marketing strategy, they supply a wide range of floral gifts that can be tied into calendars, so you don't forget those special dates.
J Parkers Long-established company supplying bulbs and plants, planting videos and garden blog.
Suttons Seeds The long-established (1806) seed company supplying seeds, young plants bulbs bedding plants and garden equipment.
Dobbies The online arm of Dobbies Garden Centres covering all in store lines plus garden club members discounts.
https://hayloft.co.uk/ Yvonne founded Hayloft from an old oak kitchen table back in 1993 in Worcestershire.
Patchplants Specializing in plants for the home and office plus gifts Online help with plant parenting and plant doctors.