The link you provided seems to say there are 3 separate plants - Buddleia tricolor is available as one plant with 3 different coloured blooms on it, so I'm not certain whether you will receive 3 plants in one pot which effectively act as a single plant because they may be impossible to separate at the roots anyway. Either way, this is a Buddleia davidii variety - that means it needs cutting down to within 2 inches of the old wood every March to keep a tidier growth habit, after the first year or so. Average height between ten and twelve feet, with a spread of up to ten feet at the top of the plant - the stems have an arching growth habit. Even though you cut it right down to virtually a stump in spring, it will put on up to ten feet of growth in a season once mature.
You can certainly plant round it, but only if you are planting into a sizeable bed, when it should be placed towards the back, but with enough room for its branches to spread out without being crushed against a fence or wall. Something evergreen is a good idea because buddleia is deciduous, and after the first 3 or 4 years, there will be a noticeable 'stump' of bare wood towards the base which an evergreen shrub will block from view, so maybe a Choisya variety if there's room - these flower around May, well before the Buddleia will. Choisya Sundance has yellow leaves; Choisya ternata 'Aztec Pearl' has green leaves, but there are a couple of other varieties - all have white scented flowers and all reach about 6 feet with a similar spread. Other options include Convulvulus cneorum, a lower growing,rounded grey leaved evergreen shrub with tubular white flowers in May, and the deciduous Spirea varieties - these usually have pink flowers in May/June and there are several yellow leaved varieties; 'Goldflame' is a good one with a height and spread of 4-5 feet, Magic Carpet is similar but lower growing. Cistus corbariensis is a hardy, evergreen plant with white flowers in May that does not get as large as Choisya varieties, making around four feet in height with a spread of up to five feet. There are other varieties of Cistus available such as Cistus purpurea with pink flowers- this is not quite as hardy, but in London, it should be fine. All plants mentioned, including the Buddleia, prefer full sun if possible.