I already have a United ticket having me arrive at LHR, Terminal 1. I am considering different options for buying an additional ticket for travel onward from there.

One option would have me leaving on BMI from Terminal 1. The other would have me leaving on British Airways from Terminal 5.

How long should I allow for transfer with either of these options considering immigration, checking in, etc.?

I am not an expert and you should check with Heathrow and your airline. Off the top of my head I think 2 hours for the BMI from terminal 1 and 4 to 5 hours from terminal 5.

OK, thanks.

I would change the ticket I already have, but it would cost more than buying a connecting flight.

The times above should be enough for the actually logistics of arrival and transfer. But, just be aware that buying the two separate tickets is different than a single ticket single ticket with a connection. If you miss that second flight for weather or other delays with your first flight you will have no recourse to get rescheduled on a later connecting flight. Not with the first airline and not with the second. And, if the second airline has a check in deadline you have to be in time for that also. So I would allow plenty of extra time.

You can't check with "heathrow" as there's no such animal. You DO need to talk to BA andBMI staff.

BMI's policy is "Passengers with connecting journeys using separate tickets are no longer being offered the facility of through check-in or through tagging of baggage" (www.bmivoyager.com/know_heathrow) That means, unless United re-issue your ticket as one UAL/BMI ticket, you have to go through immigration, collect your bags, go through Customs, then recheck in the departure area. About 2 hours.

BA's policy is broadly similar, except that you probably CAN'T get your ticket reissued. You need to check with BA, but if you check in online ahead of taking your UAL flight, you need to allow only a few minutes longer for the connection to T5 (go through Customs with your baggage, follow the signs for the Heathrow Express, get it to T5 then dump your bags at a bag drop. From leaving T1 Customs to arriving at the T5 drop, 20 minutes plus waiting time (ave 7.5 mins) for the (free) Heathrow Express to T5. Max 2.5 hrs: in practice nearer 1.5.

PLUS (here's the rub) time added on to cover yourself against a late-arring UAL flight

Thanks, Flanner. That's exactly the information I was looking for.