what could the bolt-on bolt to?
It bolts on to a PAYG plan.
As I understand, you already have an O2 sim in your Galaxy S2 which is on a Pay Monthly contract till June. If you don't renew, it will revert to PAYG, probably O2's Talkalotmore tariff. I'd recommend changing that for free to the Text and Web tariff which when you top up £10 gives you 500 MB data + 300 SMS during the following month. Then draw down the £10 top up on Web Bolt-ons for the successive months. Then reiterate.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC