That indeed does look like an RF2 :/
>> BT won't do anything either because it doesn't affect voice.
No they wont because it doesnt
>> So the simple solution should be to ring up my ISP, O2, and get them to send an engineer out, right?
That is the correct way yes.. and it is O2's responsibility as you have an incompatible product on your line for adsl. We've come across this problem before that often ISP support doesnt know what an RF2 is and how it can actually filter out frequencies needed for adsl to work properly. If they request removal via BT Wholesale it should be done free of charge by BT Openreach. They should mark on their request that there is an RF2 on the line.
Unfortunately the only legal way it can be done is by a BT Openreach engineer and therefore the site cannot condone you doing this yourself.
The Openreach engineer has a couple of options, he can replace the RF2 with an RF3, or he can by pass the RF filter..
.. to do this he would likely either reterminate the drop wire into the white IDCs' or looking at your pic, possibly take the wires from the IDCs (where the white phone cable connects) into the grey screw connectors, so that the inductors are bypassed.
Both the IDCs and screw connectors should be labelled AB so he'd need to make sure that the correct wires on the IDC's go to the right bits in the screw terminals.
Whilst some people have done this themselves, we do not condone this action, and the above is for information only. I cannot stress enough that the RF2 is BT's property and any rewiring needs to be done by a qualified BT engineer.
[edited to add]
Please note I am not a trained telephone engineer.