Functional bikes. Not Porn not Anti

Friction - £10-20 downtube shifter.

Derailleur - any old one you have knocking about, £20 for an 8 speed cassette.

You got those fancy high end fat roadie with those plastic shifters, don't need to spend as much money as you would on a workhorse.

+1 to this.

You'll probably be able to pick up a rear mech and possibly bar end or friction shifters at the pop up jumble honesty box bit too.

My 8 speed upgrade was crazy cheap - £14 for a new cassette, £5 for a chain, £7 for a pair of Simplex downtube shifters cough ignore the Middleburn Duo rings I have sat waiting for me at the bike shop cough