Like others, need to know what your goals are:
* get more AAdvantage redeemable miles (RDM) for future "free" trips on upgrades?

* get more AAdvantage Elite Qualifying Miles (EQM) within calendar 2010 to get higher status on AA for the 2011 program year?

* try out a new airline just for the experience?

* try out a new airline because you're shopping for a new airline or new FF program to give your loyalty?

Lots of different options depending on what's most important.

For example, if flying a different airline like BA is important, but you don't want to lose out on getting RDM and don't want to pay a high fare to force a connection through Canada, perhaps join another FF program that doesnt have the USA-UK mileage/award exclusion that AA/BA have.

Two that come to mind: LAN (LA) LANPass - South American airline, member of oneworld, decent but not wonderful FF program, elite on them does have some AA-specific benefits beyond the standard oneworld elite levels. Or Alaska Airlines (AK) Mileage Plan. AK isn't in oneworld but does partner with AA, BA, LA, CX and QF for RDM including RDM elite bonuses. Of them, AA, BA, and LA earn EQM for the AK program. AK also partners with several SkyTeam members, including DL, AF, KL. DL and I think KL also earn EQM. DL and AK will begin (actually resume because they had it with NW) special reciprocal benefits with unlimited as-available upgrades for each other's elites.

Neither program will boost your AA miles balance, but could become your new program you use when flying AA, BA, DL, etc.

If you want to fly Virgin Atlantic, you could credit it to the Continental OnePass account. There are some specific requirements for earning CO miles on VA, especially for EQM. Not sure of details, but they are on continental.com in the partners section. I think the Virgin flight has to be a CO codeshare, but you don't have to book it as the CO flight#. You can book it as a VA flight, but CO has to offer a CO flight# on that flight as well. I'm pretty sure all the IAD, JFK, EWR, and BOS VA flights to LHR qualify.

Also CO is now in Star Alliance, so any flights on LH, UA, US, AC, or the other 20 *A carriers can be credited to them. Or to UA. But CO offers VA as an earn/burn partner, which UA doesn't. UA offers some additional mideast partners that CO doesn't but are non-EQM earning. CO has mix/match class/award type online booking and good visibility into partner awards. UA's site can only book UA awards, and they block lots of actually available partner awards (Starnet Blocking in UA forum). But they now offer 1-way UA-only awards at half miles, and do have decent domestic award availablity.

So lots of ways to go. Think about the combo of benefits most important to you.

BTW don't rule out Air Canada via Toronto. YYZ is a good connecting airport to Europe. Well it was before the CATSA overreaction to the underpants guy. That's only on your return connection YYZ-usa. Outbound you don't even clear Canadian customs, just a cursory immigration at a special connecting passenger desk that never has more than 3 people in line in my experience. Excellent lounge - MLL in the international departures terminal. Plus Tim Horton's donuts. Earns miles RDM and EQM in either the UA or CO programs you already belong to. (the flights, not the donuts)