One more layer is the rule I was taught for clothing babies. :)
If you need three layers (i.e. top, jumper and coat) then baby needs four (i.e. vest, sleepsuit, jumper and blanket).
If you're comfortable in two layers (i.e. top and jumper) then baby needs three (i.e. vest, sleepsuit, jumper).
If you're comfortable in one layer (i.e. t-shirt) then baby needs two (i.e. vest and sleepsuit).
If you're warm and wearing the bare minimum (i.e. bikini) then baby needs one layer (i.e. summer playsuit).
If you're roasting in the bare minimum then baby needs just a nappy.
Layers can be anything, things like a wrap or a blanket count as an extra layer.
Hands shouldn't need covering once the nails have been trimmed until the winter, when if you need gloves then baby needs gloves.
Feet you might want to pop a pair of socks on, if they don't have a footed sleepsuit on. If their feet still seem cool, then put another pair of socks over the top (I use the fluffy slipper socks for this) or you can use soft baby shoes or booties. Sock-Ons can be very useful.
Hats for warmth shouldn't be necessary after the first 24 hours or so at this time of year in a standard UK climate, unless you love hanging around the freezer aisle at the supermarket. A sun hat however is a good investment. I recommend ones you can tie on!
One more layer also applies for swimming (if anyone is planning to do so) so baby will need a costume and then some type of wrap or wet suit to keep them snuggly warm if you're using the adult pool. Some leisure centres have extra warm baby pools, where the extra layer is not required.