I signed up with 888 using a similar offer a few months ago - you got a £10 free bet for every four £10 qualifying bets placed at odds of 2.0 or greater.
With a total bonus amount of £100 available, that meant 37 qualifying bets were needed.
However, I got restricted after I'd placed about 10 qualifying and free bets. Following that, trying to find any bets where 888 would accept a £10 stake became increasingly difficult.
In practice, the only qualifying bets available were on English Premiership games where the odds were between 2.0 - 4.0, and perhaps the occasional Scottish or foreign match. It also meant that getting a decent return on the later free bets took a bit of effort.
Perhaps I was unlucky to get so restricted, but from my experience I'd have to say go for this latest 888 offer but be careful.
That final £18 free bet looks attractive, but if 888 only allows you to place such an amount at very low odds (if at all), then the cash that can be realised from it is dramatically decreased.