OM has a risk free equal profit calculator which allows you to lock in a guaranteed profit, I use that in the circumstances you outline above. Bet Victor have often offered me something similar to the above.
You need to use a calculator to work out the lay stake so that you can guarantee a profit from this offer. You would do this by doing what we call an underlay. If you don't use one of the MB services such as OM or PA then you could use the free Matched Betting calculator at Trickybet...
I think @homie has given good advice on the way to approach it in terms of which option might be best suited to your circumstances, there is an option to request a change to your selection if you're not happy with your initial choice anyway.
I see Ladbrokes have been splashing out again this week with their £1 free bets....think I have had three so far...pushing the boat out.
As far as the Rugby World Cup is concerned I think there might be more offers as the competition progresses. It lasts for six weeks in total!