duty free shopping

Depends which Terminal you'll be at.
There are 5 at Heathrow.

It's really very simple.

Heathrow duty free is guaranteed to be at least as cheap as the city centre prices (in Britain) the chains concerned would charge. MINUS VAT and excise duty. There's no import duty in Britain on most things, and it's trivial compared to the US on the few things where it's charged, so the common myth that duty-free shops work because you don't pay import duty is just wrong.

Since there's no excise duty on anything but booze and tobacco, and foreign residents can get VAT refunded, there's no obvious financial benefit for most visitors to buy duty-free. Whether, in practice, there are bargains or unique products though depends on which shops you're visiting, where you live and the state of your currency against the £.

Sales per square foot at LHR are about the highest of any kind of retail business anywhere on earth (so forget all that crap about ripoff prices), so the shops obviously meet some customers' needs.

You may visit ONLY duty free shops in your departure terminal, and they all have mini websites on the Heathrow site.

Go to those sites and see what's on offer. Oly you can decide what's worthwhile

Arriving at Terminal 4 and want to go to Terminal 3 - have about 1 1/2.

Oh OK. Just got back 2 days ago from a multi-flight trip and flew into and out of Heathrow a few times but only through terminals 5 and 2, so cannot help you with 'been there' accounts of Terminals 3 and 4 as I'd thought I might. Flanneruk gives good advice to checkout the on-line sites for whats on offer.

While I didn't have to reclaim my luggage between terminals, I did have to go through full security checks (coats, belts, shoes, laptops off) inbetween and if its busy as its likely to be (though the line moves efficiently and quickly), this could cut into your browsing time as it did for me.

I didn't have time to check out T2 shopping (I too had a narrow flight connection) but did have time the 3 times I was in T5 to see whats was available and only bought stuff on my final flight home. I noted that the prices on the tags at the Harrods store indicated the approximate discount you were getting compared to if you were purchasing at the main store in London. I found 2 shirts I liked on sale at the Thomas Pink store that were advertised as an airport promotion.
There was also a chocolate store that sold Thorntons and various other confectionary : creamy chocolate bars for under a pound each, and many other miscellaneous stores for electronica and drugstore stuff. All of this, more or comparable should be available in your terminals.
Theres even a Gordon Ramsay restaurant in T5 (dine in or take on to aircraft) but not sure if he has them in the other terminals.

Well Terminal 5 is basically a shopping mall with an airport attached The shopping starts behind the check-in counters and continues almost as soon as you step through the security x-ray machines. It continues on both levels and, to a lesser degree, if you move out the to "B-annex" gates. The selection of shops is not bad--as these things go--but you almost have to dodge the shops to reach your departure gate. (Nice food choices at the new terminal though).

If you want to see an exhausting, er, exhaustive list of Heathrow shops at all terminals, try this:

http://tinyurl.com/LHRshops

Thank you to all - very much appreciate it.

The other thing you should be wary of is to where you are flying. If you are flying non-stop from Heathrow to your final destination, then liquid bans won't factor in. If, however, you are connecting in the US, this creates hassles.

For example, if you are flying LHR-JFK-DEN, then you would need to go through security in JFK. Since you can't carry anything other than your little baggie with 3 oz liquids through security, then you will need to check the liquids from JFK-DEN. Personally, I find this to be too much hassle and don't bother with liquids under such a scenario. If you are flying non-stop to your final destination, then you have no such concerns and you can buy liquids without worry.

I mention this because I think that liquids (specifically alcohol) is where the duty free at Heathrow excels. Particularly for whisky, you will find the selection excellent and better, even, than many liquor stores. The fragrance and cosmetic shops also offer an excellent selection, though perhaps not much better than what you might get at Sephora or a good department store.

The rest of the shops are less fertile hunting grounds. The electronics might be worth a look for certain travel-related items like adaptors, but if you live in the US, your typical Best Buy will offer far more selection for significantly less money. Otherwise, you have some clothing shops that would only be worth a look if there is a specific label that you like but have trouble finding at home. But, as with electronics, don't expect significant bargains compared to US prices, for example.

Beyond that, you can always stock up on the various knick-knacks that you would expect from a tourist shop, like UK-themed stuffed animals, pretty packages of tea, and all manner of stuff emblazoned with the Harrod's logo.