have a look on the Halfords website, over the past year they have had some fantastic deals from time to time on own-brand and on Mobil oils; sometimes on the 4-litre and sometimes on the 5-litre. Mine uses synth or semi, which is a bit expensive.
I got some fully synth Mobil so cheap that it was not worth buying a 1-litre bottle to keep in the boot for top-up, just got a big one. You can check local stock, and order if necessary
Their own-brand seems to be made by Comma, if you click the link to their Safety sheet.
When there isn't a special offer, Tesco is cheaper, or Wilkinsons for own-brand and Castrol only, who again will order free for collection from your local shop.
In my case, the branded maker's filters are so near in price to substitute brands, that I get the genuine ones. A local motor factor, or online supplier, will usually be cheap. I last got 2 oil filters, 2 new drain plugs with washers, 1 air filter, 1 cabin filter to save paying multiple postage fees.