Can anyone tell me what type of snake this is?
I believe this is a Barred Grass Snake, Natrix helvetica. There are only a few snakes naturally found in the UK, and the Grass Snake matches the size, color, and pattern (bars on the sides but not on the back). The "N. helvetica" designation is relatively new, 2017, following genetic testing that showed that the previous name of Natrix natrix was not correct. "The newly distinguished snake is grey, not olive green like the N. natrix and does not have the same bright yellow collar." 


Youtube video
Stock photo from Alarmy
Publication describing geographical distribution: Phylogeography and taxonomy of the barred grass snake (Natrix helvetica), with a discussion of the subspecies category in zoology. Figure 1 (below) includes a photo




Smooth Snake is another possibility but seems much less likely to me. 


  Smooth snakes are slender and normally grow to around 55 cm (2 ft). They are grey or grey-brown with darker markings along their back and usually a 'butterfly' shape on the top of their head. They are very rare in the UK but can be found in heathland habitats in Surrey. It would be very unusual to find a smooth snake in your garden. They are very secretive and would normally be found underneath objects.


--Which snake is it?
 (My emphasis)