Be careful you don't end up chasing the marginal gains. Turning lights off, great idea. It does save energy. But, emotional health can be improved by being in a well lit space. A typical LED light bulb these days is about 9w. You need to have that bulb on for over 111 hours for it to cost you 1KwH. Not stubbing my toe in the hallway (light switch is far end of it, and only one end), is worth having the light on for. The same with the whole "Make sure you switch your TV off completely, not just on standby". EU law requires that devices like TV's consume 1W or less on standby. That's gonna take you almost 42 days for it to use 1kwh. Worth it for the improvement in quality of life of being able to turn the tele on from under your warm blanket...
So, rather than worrying about marginal gains, look at the big things.
Don't use a tumble drier. Unless you absolutely need to have your clothes dry in a couple of hours, just hang them up, outside if it's dry, inside if not. This should save in the region of 4-5kwh per cycle. Depending on how often you use your tumble drier, that can add up. Certainly faster than turning off lights, switching the tv off at the wall, or faffing about measuring only the amount of water you need for your tea.
Check for drafts. The highest standards of energy efficiency basically require your home to be a sealed box. With very little air exchange. Pretty much every home not built to these standards is going to be a drafty mess. But you can greatly improve things by those draft excludy strips along doors, checking windows etc... If you can keep the exchange of warm air indoors, with cold air out, to a minimum, you'll improve things a lot. A good pair of curtains can really help to reduce losses at the windows.
It can be a good idea to get heat exchanging vents tho, these can recover about 90% of the energy from the air they vent out, while warming the air that comes in, all for about 10-30w of energy.
It's very hard in a typical British home to be more energy efficient than most people already are, without drastically impinging on your quality of life, or without spending a lot of money on new things. Most of the common advice people give for energy saving is going to save pennies when multiplied out to a whole year. Even the thing with boiling only the water you need is very much in marginal gains territory. It takes about 0.046kwh to boil 500ml of water, and 0.091kwh to boil 1l of water. If every time you want to make 0.5l of tea, you boiled 1l instead, you'd need to do that nearly 22 times to cost an extra 1kwh. (exact numbers may vary depending on efficiency of your kettle). But. If you boil an extra 0.5l of water, you're not pouring it down the drain after you've made your tea, it sits in the kettle to be reboiled the next time (after you've topped it up again no doubt). As that water cools, the energy from that water is going to dissipate into the local environment. It's going to warm your kitchen a little. It's a marginal gain, for a lot of extra faff when you just want a cup of tea. Obviously if you drink a lot of tea every day, that may have a more significant impact over the whole year. The UK tea and infusions association[1] says the Brits drink 100000000 cups of tea a day, that's less than 1.5 cups per person per day. So that 22 cups of tea is going to work out at an extra 1kwh every 2 weeks, or 26kwh over the year. Or about a fiver...
J
[1] yeah, I was surprised by that one too...