This is my revised version of the summer synoptics guide.
The standard summer synoptic setup
Traditionally, in summer, we have a strong Azores High out to the south-west, low pressure systems moving from west to east to the north of Britain, and westerly winds dominating, bringing cool cloudy weather and rain at times. Southern areas see the warmest and sunniest weather as they are closest to the influence of ridges from the Azores High.
Low pressure dominated scenarios
Cool, cloudy, unsettled summer weather is often associated with a conveyor belt of strong westerly winds, a flattened Azores High well to the south-west, and low pressure systems and fronts bringing bands of rain west to east at regular intervals. The Julys of 1992, 1993, 1998, 2002 and 2004 all had this pattern. When lows track further south than usual (over northern Britain, say, rather than to the north of Scotland) it can be especially wet- July 2007 was a good example.
However, if we get a significant gap in between fronts, the result tends to be a mix of sunshine and showers- the most common pattern being a sunny start, a build up of cloud towards the afternoon and then sharp showers. When we have slow moving low pressure close by and no frontal activity, such "sunshine and showers" weather can persist for days on end- this situation generally arises when the jetstream is weak.
However, if the low pressure is also associated with slow moving fronts, then instead of being bright and showery it tends to be cloudy and drizzly.
High pressure setups
For spells of warm dry sunny weather, many look out for ridges from the Azores High extending over towards the British Isles, quietening the weather down.
Sometimes this can indeed herald the start of a fine spell, if high pressure can establish over the British Isles for upwards of a few days, but more often, the ridge brings just a day or two of fine weather before the next Atlantic system comes in and the high retreats to the south-west. Often the ridge just covers southern areas, giving warm dry sunny weather in the south, and dull damp weather in the north.
If a ridge from the Azores High connects with high pressure over and/or to the east of Britain, however, we may get a prolonged spell of warm dry sunny weather, for instance the famous summer of 1976 was dominated by this setup.
Blocking over and/or to the east of Britain can sometimes bring hot sunny spells on its own, without the need for ridging from the Azores High- such a pattern typically has low pressure to the west, and the Azores High displaced to the west of its usual position.
Persistence of this pattern resulted in the hot summer of 1995 and the exceptional July of 2006. It can also give rise to significant thunderstorms when Atlantic systems push against the block, bringing a "Spanish plume" event with southerlies bringing storms up from the near Continent. However, if the jetstream strengthens, such a pattern is usually temporary, as the Atlantic systems push through, the block retreats eastwards and we get a thundery breakdown followed by westerlies.
Sometimes high pressure prevails to the north of Britain (this type of setup is far more common during June than July or August, as the westerlies are traditionally weaker). This brings a pattern of easterly winds, it is often warm, dry and sunny in the north-west, cool, dull and misty near the east coast, while central and southern areas tend to be warm and humid with thundery rain periodically moving up from the south.
The mid-Atlantic high
Finally, it is also possible for the Azores High to be displaced northwards into the mid-Atlantic, giving northerlies over the British Isles. This setup tends to be cool and cloudy, especially in eastern areas, as frontal systems move southwards around the periphery of the high, though western areas are often sunny.
However, if we pick up an unmodified draw of air from the Arctic with a significant gap between fronts, the result tends to be sunshine and showers- similar to what I described under the low pressure setups- this setup tends to be cool but can also provide very dramatic weather with hail and thunder, particularly for eastern England.