The JISA at Interactive Investor (II) is free if you have a trading account there (in addition, you also get a ISA free for that). The trading account is £9.99 a month. Of course you still have the cost of buying a fund … but II makes it free for a regular monthly investment.
The issue more is your ROBO requirement … You could buy any fund or go for one of their model Vanguard portfolio’s (there is an extra cost for those … I don’t know how much)