You really don't need a lot to get going at all. Until the start of 2015, for 13 years I cycled 15 miles, 3 or 4 x a week for horse riding fitness.
Like doing anything outside in the UK you just need to be warm enough and dry enough, it doesn't need to be cycle specific at all - base layer, fleece, waterproof jacket and gloves - any old stuff you have that works will do.
Essential bike specific stuff - Hat, Hi Viz and a puncture repair kit/pump (was still using my 13 year old one until this year (smile)) and padded shorts - my 13 year old shorts are still going strong. Maybe a small multi tool.
I never used my lights because I didn't cycle in the dark, any cycling specific gear I had was a DH cast off because I wasn't into it enough to spend a lot of cash on bike gear.
I stepped up my cycling at the beginning of 2015 and now have lots of additional stuff but cycling has replaced horse riding for me now that my horse has retired and until I was downed by an op recently I had been managing 3 x 30 miles a week, 40 on a good day if the sun was shining and DH was with me.
Beware of deeply pad saddles/gel covers, they allow your seat bones to sink into them which means that more of your undercarriage is in contact with the saddle, be it gel covered or not.
I use leather saddles, your seat bones are supported by the saddle - less bodily contact with the saddle. I have never used chamois cream but did feel sore if I did my old 15 mile trip a few days running before I switched to leather.
You can spend as much, or as little as you like I suppose but you don't need all the gear you can buy.