Each of you should take a $50.00 bill with you with one of you buying coffee at your departing airport. Leave coinage in the tip jar. There's your emergency float which will most likely remained untouched until to return to the US and you buy another cup of coffee.
Use bank ATM's in London to withdraw your pounds and off you go. As far as what you'll spend per day all depends on what you are going to see.
Admission prices are steep for St. Paul's and the Tower of London but investigate getting the National Railway 2 for 1 deal. Use the search engine on this website to read discussions about this and if you decide it works for you do not forget to take spare passport size photos with you.
Most of the museums are free.
Get sandwiches, fruit, crisps, salads at places like Pret a Manger or Marks and Spencer or Sainsbury and picnic. The parks in London are lovely. Enjoy them. You can also get great food at street market stalls or food trucks which have gained popularity in recent years.
London is expensive but given you aren't eating pricey meals , have already booked your hotel, will, I hope invest in an Oyster Card while in London, I'd calculate 100 pounds a day for u both. That's about $150.
Have a pleasant visit to both cities.