I would be very cautious about putting high value goods in hold luggage for two reasons:
1) Hold luggage is usually X-rayed these days and so the luggage handlers are going to know you have them in there - not everybody is going to be able to resisit the temptation that presents, Also bear in mind that as you are transiting through Brussels your cases are going to be passing through the hands of three lots of luggage handlers.
2) If it is not clear what they are then the US security staff are going to open your case anyway to make sure that they are not a security threat.
3) You are only allowed to bring into the UK from the US goods to a maximum value of £390 per person and there is growing evidence that HMRC are doing spot X-ray checks of incoming baggage of passengers originating from outside the EU and calling passengers over for manual inspection of cases suspected of carrying goods in excess of this limit. They are entitled to confiscate goods over the allowance without even giving you the option of paying the VAT and you could beliable to both confiscation and a fine.
4) As your first contact point with the EU will be in Brussels then you might have to go through Customs there before boarding your flight to the UK as well and so your cases and hand luggage could be subject to two lots of customs checks.
Far be it for me to suggest ways in which you could get away with bringing in more than you are allowed but having them in your handbaggage means that it will only be inspected for security purposes and on the whole the staff looking at such goods are going to be concerned with the security aspect and not whether you Re trying to evade paying UK duty. After all, for all they know you could be planning on going through the Red Channel and declaring them to HMRC. Speaking personally I wouldn't take the risk - I couldn't afford to have them confiscated but others think differently and have successfully taken the chance. Only you can decide how much you would be prepared to lose if caught.
On the otherhand, if the tablets and kindles plus anything else you are planning on buying and bringing back with you actually total less than £390 per adult then do make sure that you have the receipts handy to prove this because otherwise if you are the subject of a spot check then HMRC will assess their value for themselves and they could potentially decide that you are over the limit even if you actually paid less.
SM