Mediterranean basil is easily available and I have successfully grown it many times; however, my enquiry is twofold regarding 'Holy basil'. I first came across this herb whilst in Thailand. It is has an unusual flavour that cannot be replicated. Like so many Thai ingredients, it is readily available there as it grows in the wild under many different conditions--wet/dry/hot/humid. The closest I have come to trying anything similar was a 'lemon flavour' variety of basil, which unfortunately did not germinate. I was very disappointed; however, before I try this variety again, is it possible to locate and purchase the seed of the Holy basil plant here in the UK?
You should be able to buy seeds for this in the UK. You might want to look for it under the name tulsi, which is another common name for it.
This is one site that sells the seeds: http://www.chilternseeds.co.uk/item.php?id=1376P
We live in Victoria, bc, canada
It needs a warm climate so you can grow it in a container and move it inside for the winter. Here is info on growing seeds. You can start it indoors now and move it outside when the danger of frost has passed.
https://www.gardeningknowhow.com/edible/herbs/basil/holy-basil-uses-and-growing.htm
https://www.gardeningknowhow.com/edible/herbs/basil/growing-basil-seeds.htm