Oats can be good source of body in a small beer, as can Torrified Oats. I've found that a pound will add body and allow you to dry the beer out a little more, without making it sweet, which increasing the Crystal Malt can do. The British brewers that I've talked to have generally used Crystal malts in the 2.5-5% range, when they use them at all.
Other character malts, like Amber Malt and Mild Malt, can add complexity that will help make the beer seem bigger.
On small beers, be careful about extra sparge water. Tannic qualities in the wort will quickly show up as astringency, which can make the beer seem watery.
Historically, British beers tend to have hops at the equivalent of 60', 30' and near flameout, or some combination of the two. However, they are usually allowed to steep on the flameout hops for a while, which makes for much more flavor than you would usually get from late hops with the usual homebrewer's rapid chilling techniques.
For me, the best plan would be to try to find a 3.5% mild recipe or something. If I could make a full bodied, good tasting 3.5% beer, that's really all I want.I've been on a similar quest recently. I think the key is to keep the sparge runnings high in gravity (or do a no-sparge mash), mash for moderate-to-low attenuation, and use a yeast that will contribute a lot of esters.
I've been lucky to be in a position to get a lot of very fresh British beers. I've had 100-200 over the last 5 years, and another 20 that I had never had before, just last week.
I have a real fondness for 3.2-4.2% Bitters. A lot of them are very light and refreshing beers, which I've learned to appreciate. Still, it amazes me that many of them have quite a bit of body, but have very little to no Crystal Malt and are mashed at 150°F. There are occasional sweet ones, but most of them are as dry, or probably drier, than most US beers, but with a softer malt character. Another trick is to keep the carbonation low, since carbonating a low gravity beer too highly can really cut the body and turn it into soda pop.
While some have subdued esters, the yeast is a big part of the best of them. A strong ester character does a lot to make a beer seem bigger. I taste WLP002 character in some of them, but a lot use much more characterful yeasts.
My experience has been that it's not too difficult to get a very tasty beer down to about 3.5% with a little creativity and attention to detail. It might require more recipe development to get something you like below that.