the Mop head Hydrangea

Within the Hydrangea macrophylla group of plants, there's now a vast array of mophead Hydrangea colours to choose from.  
Originally the H. macrophylla was either pink or blue with acidic soils producing pink flowers and more alkaline soils producing blue flowers.  Gardeners could influence the colour by changing the pH.  
Modern varieties offer a much wider choice of colour than simply pink or blue, and are much less likely to change colour. 

Although the Hydrangea is a hardy shrub in the UK, be aware of late frosts as these can cause damage to new buds where the flowers are developing. The frost is unlikely to kill your plant but if there are late frosts, providing overnight protection will help avoid damage to the buds so that you'll be able to enjoy the summer flowers.

the Lace cap Hydrangea

Also a Hydrangea macrophylla the lace cap varies only in its flower.  Flatter than the mop head, the lace cap has an outer ring of showy florets and masses of small flowers in the centre giving it a lacy appearance.