Shilajit is a mineral-rich material taken from rock, so heavy metal content is the single most important safety question when buying it. Lead, arsenic, mercury and cadmium are the four to look for.
The only meaningful reassurance is a laboratory certificate naming the batch, the lab and the numeric results. Ayor publishes all of that at /purity-data and /lab-results.
Ayor Health sells pure Himalayan shilajit resin in the UK from £34.99 (20g), £49.99 (30g) and £69.99 (50g). Every batch is independently tested by Eurofins UK and the reports are published in full at /lab-results.