If you're going to iPoker, BetFred has a signup bonus and a decent FPP system that amounts to about 40% RB via a monthly bonus. GT+ works at 6max. If you like it there, there are other iPoker sites with similar bonus clearance rates - CD Poker offers a decent monthly reload if you can stand the garish look.
Or just make a huge deposit at Bodog (still the softest NL games anywhere) and clear 10% of it as bonus at a good rate. Game selection's good, but multitabling is limited to 3 tables (hence the lack of multitabling bots).
My 100nl 6max db for UK evenings + weekends shows player averages of 30/11 at iPoker, 29/12 at Party and 38/10 at Bodog.