There is no equivalent service from US to UK.

We call it 'signed for'. You call it registered.
International from UK, you can use that in conjunction with any delivery service, at an add on cost of £4.20 (£4.20 is my account price - may be a little more at the post office) and that includes £250 ($400) insurance. Extra insurance can be bought for, I think, up to £2,000 ($3200). Doesnt cost much. Something like £2 or £2.50 more.
So you can add that to regular mail, or priority mail, for letter, flat or packet.

Now from the US, although the barcode (or number usually starting LC) is scanned from your green customs sticker, it is not trackable.
Trackable numbers are only found on the customs papers required for packages 4LB and up, and/or the medium and flat rate international flat rate boxes, so on that basis $45 approx is the cheapest trackable alternative.

The express/overnight envelopes are trackable but Im not sure they will allow anything other than paper in them. In any case, if you had such a need to send a single chip or two, then fedex is slightly cheaper.