The portrait does look like Vespasian, but Titus and Domitian are very similar.
The reverse is an Altar, and according to my books that makes Domitian more likely. With so much damage, it is difficult to be sure. Most likely minted in Rome but for use across the whole empire, including Britannia. The closest match I have spotted is S2676 which was minted with Domitian as Caesar under Vespasian and would therefore date 69 to 79 AD. (But I stand to be corrected!)
Does appear to be silver. The mass devaluation of coinage and the use of progressively less and less silver had not started in the first century.