my vote goes to Day Nurse and Night Nurse. If you go to Boots they have the exact same stuff for a fraction of the cost under the Boots brand, just ask for it.
You mentioned allergies and we use Zertec which you can buy over the counter here but I believe you need a prescription for in the US. Again Boots has their own brand, the active ingredient is certirizine hydrochloride. This is what was recommended by a pharmacist friend in the US as being the best.
Zyrtec/Zirtec is one brand that is the same in the US and UK as as far as I am aware is available off the shelf in both countries - same with Claritin/Clarityn. What confuses me is that here in the US, Zyrtec is advertised as working 2 hours faster than Clarityn but can cause drowsiness. However, having worked behind the counter for Boots for a number of years, I can tell you that both are sold as non-drowsy antihistamines (and contain exactly the same ingredients as in the US) and neither technically works 'faster' than the other, it's just that some people find one works better for them than the other. Personally, I have tried both, and to be honest, it didn't appear that either of them did much for me!
The main difference I have found is that US Benadryl is different from UK Benadryl. In the US, it contains diphenhydramine hydrochloride (also in Nytol and Nightnurse) and is drowsy, whereas in the UK, it contains acrivastine which is generally non-drowsy.