I have a platform called Mytrack edge. which has options futures and basically about anything u need built in. The options work from the FTSE cash price.
To work out the settlement price for FTSE100 cash, it is not the last price but the settlement price Exchange Delivery Settlement Price,EDSP its worked out according to a formula which tries to prevent manipulation.
the settlement price normally can be found here
UKXSP.L Technical Analysis | FTSE 100 EDSP Stock - Yahoo! UK & Ireland Finance
todays appears to be 6166.21 so all calls at and above this level will be worthless and all puts at or below this level will also be worthless.
I have never found the open interest at various strike prices to be predictive of ftse moves, an old wives tale is that the market moves to the point of biggest open interest on expiry, the best way I find to use this prices is to have your very own FTSE vix, so today the 6150 call and 6150 put trade for 61/69 total of 130. so the market is pricing in that FTSE during the next 30 days will trade in a 130pt range. this is pretty reliable. So if you happened to be long you would have a general idea not to close your long until u saw maybe 120pts range .
Options are usually traded in combination, people sell one option to finance another.