You might look at a day pass in one of the "clubs".

Here's a map of LHR. Click for each terminal

http://www.heathrowairport.com/heathrow-airport-guide/airport-maps.

You are also coming from a non-Schengen country into a Schengen country, which means you will have to go through passport control.

You also can find terminal connections on youtube.

Hopefully, your luggage will be checked all the way through so you don't have to lug it around.

"You are also coming from a non-Schengen country into a Schengen country, which means you will have to go through passport control. "

You're not, and you don't. At least not in London.

If this booking is on one ticket (typically if it's all on one airline or one airline alliance, and you booked it all at once), you go airside from T1 to T3 (just follow the signs) without going through immmigration, allowing time for going through security again (NO-ONE gets on a plane in a UK airport without going through security in that airport). Your first encounter with an immigration official will be in Frankfurt

If it's not, you need to go through immigration, collect your bags, walk to T1 and check in again

If your luggage is checked through to FRA then you follow the signs to "Flight Connections" and let them deal with you.

If not then you have to go through immigration, collect luggage, change terminal and check-in again.

Read the following

http://www.heathrowairport.com/heathrow-airport-guide/travel-between-terminals

Well at least you have enough time...I've seen some short connection times booked at Heathrow, which is a huge airport. You can also see info on connections explained on this British Airways site:

http://www.britishairways.com/travel/london-heathrow/public/en_gb