For your valve seats as posts above, if and when they get bad get the head converted to unleaded, some say dont buy any additive, just keep the money in your pocket and by the time you have any valve seat issues you will have saved far more than the conversion costs,
My unleaded conversion cost on a loose head £200 inc seats/ valves/guides,
The drawback is that it leads to 'as you have the head off why not do the rings' etc,
As you are in london i presume the monthly milage is low so its up to you whether fuel stabiliser is worth putting in, the new petrol goes off, buy a good name product.