Is not so much that firms like Animed are cheap; it's more that most vets are ruddy expensive! (Two examples I've noticed in the last week of prices at my local vets: Bags of 'Dreamies', which are usually on offer at £1 in Asda, Tesco or even the Co-op, are priced at £1.67 at the vets. Plastic pet beds, similar to those sold in Poundstretcher for £2.99 , cost £9.92 at the vets).
Animed (which is based up the road from me) is a well-respected and reliable firm, with nearly 40,000 reviews on TrustPilot (scoring 9.6 out of 10):
https://uk.trustpilot.com/review/animeddirect.co.uk
There's no reason not to use their services.
However if you'd prefer to buy over the counter (without paying the high prices that your vet charges), look no further than the pharmacy at your Asda, where they sell Adocate and other pet treatments at reasonable prices.