$465 MSP/LHR
I would book MSP/DTW now and secure the "old" pass to use within the next year, using your buddy's card, for the April trip. It will cost them $25 to change to MSP/LHR + mileage--total $490. He won't have to pay the $465 until he flies but you aren't burning a new pass for him.
We do have service ATL/LGW, and taxes are slightly lower. (Easter is April 8 BTW)

Here's a tip: when you want to find route info--go to the interactive map on either delta.com or travelnet--zoom in to destination city, click on it and it will show all DL flts. That way you can find alternatives to connect if your preferred gateway is sold out/overbooked.

It's a great deal to use the old ones! I book all 8 at once (that takes some time!) Make sure you have legal name, accurate DOB, etc (as on passport) because you can't change that part once they "expire". As in Buddy A and B each get pass on hold. B doesn't want to use his, you can't reissue it to another Buddy. Buddy A wants another one, you have to use a current year pass.