Here is one plan some ER folks follow. You can decide whether it is a good fit for you:
1. Determine your bare-bones retirement budget. That is, the minimum things you need for basic but decent survival (housing, food, utilities, transportation, etc.).
2. Create a "guaranteed" income stream to cover that bare-bones budget forever. This income stream should be nearly impervious to any kind of economic conditions or downturns, and it should be adjusted for inflation over the years. Think super-safe pensions, purchasing a lifetime annuity from a rock-solid company, or even a pile of cash in the bank if you're super paranoid. It would not be stocks or other volatile investments. Then, no matter what happens economically, you will have peace of mind that you always have enough to at least live a comfortable life, and won't be hungry or homeless.
3. Determine your desired "everything else" budget beyond the necessities (travel, entertainment, hobbies, whatever), and create some income streams to cover that budget. This could be more volatile investments like stocks, real estate, etc., that are expected to grow healthily and provide income over the long haul. Here, the 4% SWR is a good starting point for how much money you will need to cover those expenses, assuming you maintain an asset allocation with a decent amount in growth stocks. If your investments do well over time, you will always be able to enjoy a great retirement. But worst case, even if they don't do well for whatever reason, you always have your basic expenses covered as described above, or you can seek other sources of income (like part time work, for example ) to cover your desired lifestyle beyond basic needs.
4. If it helps, you can also break things down into periods of your life. For example, if you know you will receive the UK equivalent of Social Security in your old age, you can factor that income into your overall plan. Just come up with the different amounts you will need to live on during different periods in your life, and what income sources you will tap to cover those needs. I've done this for different periods in my life: right now while I have children living at home in a high cost of living area; the period where they'll be away from home and in college with tuition/boarding costs; the time when we may leave the high cost of living area and sell our home; the time when I'll start receiving a pension; the time when I'll start receiving Social Security; etc. I've earmarked various assets to cover each of those periods.
