3 day travel and sightseeing/tour pass for London?

Where can I get the cheapest deal on a 2 or 3 day travel and sightseeing/tour pass for London? I’ll be in England for 5 nights, 6 days at the end of June. I’ve looked on google and they are quite expensive, as I thought they would be but any other recommendations? Thanks

cost £7.. each for unlimited travel on London underground and buses for 24 hours available from any bus station newsagents or underground railway station

Mango Tours travel segment, this weeks topic “Travel Deals”