6 hours at Heathrow between planes

no.

also say no, but plenty of time to take the short taxi ride over to visit Windsor Castle

Only before 9/11 happened, now you would need time to get back through security. The Windsor suggestion sound like it might be a good alternative, but plan your return time accordingly.

I echo the above "no" answers, even if you actually have 6 hours. Does your 6 hour layover include the possibility of a late arrival, the time it takes to disembark the aircraft, the time it takes to go through Passport Control at Heathrow (always a lengthy process for non-EU passport holders), etc., etc. Then you will need to check in for your flight to Spain,(first having to go through security) at the very least, an hour ahead of departure time and it can take up to 20 minutes to walk to certain departure gates at Heathrow. Taking into consideration all of this, your actual free time could shrink to about 3 hours or less.

Not a prayer!

No way, No how.

See my response to your Transportation posting.