My thinking is that being in the UK I can play while it's 4am or something in America, so is there a guide as to when players will be more or less alert, and hence play better or worse?
Americans can't play much online poker since Black Friday because it's illegal for banks to process transactions to or from online poker sites. There are a few sites, such as Merge and the like, which will allow US players but they are relatively low volume compared to PokerStars and Full Tilt.
If you're looking to play the highest amount of fish you should target Europeans and Russians. Play during their evening, especially on Friday/Saturday nights. Weekends are 24/7 action as well.
Really, this question is far too broad since it depends entirely on your location, availability, and so on.
Every Person has it's own daily phase of best concentration. For most of them it's in the morning after breakfast, for some others it's in the night when everything is dark and silent. Both periodes make sense, figure out when you have your personal best and play at this time. Anyway playing a lot of poker will help you because situations repeat and you will be faster in doing the right choices.
I found a quiete interesing article about that topic :
I'm an American player who used to play when it was legal and I look forward to the United States getting real and making it legal again. I used to sleep during the day and play at night and drink coffee to be extra alert. I might be unusual in that way, but it can pay to be unusual, different and unpredictable in this way. Be careful of others who want you to think they are tired at odd hours.