According to Associazione Verace Pizza napoletana,AVPN

The list of ingredients on that can be used on a pizza are as follows

San Marzano tomatoes – which grow on the volcanic plains to the south of Mount Vesuvius

Mozzarella di Bufala Campana, a protected designation of origin cheese made with the milk from water buffalo raised in the marshlands of Campania and Lazio in a semi-wild state.

Caputo flour – the easyist to get hold of in the UK
Agugiaro – 5 Stagioni flour
there are others to consider

There are many approved suppliers the you can use but the probably the biggest factor in producing a great pizza is going to be the flour that you use then followed by the tomatoes and mozzarella. Concentrate on getting you base great first before shelling out large amounts of money other ingredients.

The flours that have a lower protein content and therefore lower the gluten,are the best for pizza, which give it less elasticity. Gluten levels are controlled by selecting different strands of wheat, so going for a flour that is made for pizza “00” will give you the best results.

High-gluten bread flour is made from wheat that has 14-15% gluten.
But “00” has just enough, but not too much, stretch at 12.5% gluten.

Also freshness of ingredients in your pizza is a big deal so although the mozzarella may be the exact one specified by the Associazione if it is shipped to the UK then the quality will wain. Try and get some thing that is local and fresh and tastes great. Rather than the correct type !

Basil

Fresh Basil must be used, this can be grown your self for best quality

Grated hard cheese

I usually use Parmigiano-Reggiano but it has to be from the official suppliers list to be correct which can be found here 

Olive oil

This is specified as Virgin or Extra Virgin again needs to off the official suppliers list to be correct which can be found here 

Sea salt

Must be used rather than table salt

So once you have all these !

To create a “Pizza Margherita” the specification for this are as follows 

Canned peeled tomatoes 60 – 80 g
Olive oil (Virgin or ExtraVergin)
4 – 5 g (variance of +20% tolerated)
Mozzarella/Fior di latte 80 – 100 g
Fresh Basil A few leaves
Hard cheese (grated) 10 – 15 g
Salt As needed