Welcome to the forum!
The question of early retirement portfolio safe withdrawal rates (SWR) is one that I and a few others disagree with the majority on this forum, and your specific situation is the perfect example: The oft-cited "4% SWR" that many on this and other sites quote as gospel was the result of research by a few people to find out what amount ANY retiree could withdraw from a diversified portfolio of investments at ANY period in history, and still not run out of money (including retiring in 1929 on the eve of the Great Depression). They studied the stock market returns from 1890 up until today, in 30 year period timeframes, with periods starting each year, to see what they could spend each year and not run out of money:
http://www.fpanet.org/journal/CurrentIssue/TableofContents/ASaferSafeWithdrawalRateUsingVariousDistributions/http://www.retailinvestor.org/pdf/Bengen1.pdfThe 2 big problems with this 4% SWR:
1 - it's based on a 30 year period. If you start year 31 with just $500 to your name, it's considered a 'success', since most retirees pull the plug at 65, and they limited their initial study to a 30 year time period.
2 - it's based on US retirees. When research is done on other countries, the US actually comes out almost at the top (only Canadians had a higher SWR). Nearly all other developed countries in the study had lower SWR of 3.5% down to as low as 0.5% (Japan). The 4% SWR assumes you spend part of your portfolio in some years (depending on how stocks/bonds do) along with interest/dividend income.
Here's a link to a study for other countries:
http://www.fpanet.org/journal/CurrentIssue/TableofContents/AnInternationalPerspectiveonSafeWithdrawalRates/Subsequent research has yielded that a retiree looking at a 40-50 year retirement must have a considerably lower SWR for the same 'safe factor' (unless you are ok with slashing expenses and/or looking for work). I don't recall the number, but using US data, I believe the 40 or 50 year SWR was a hair above 3%.
Personally, I'm hoping to retire at about 45. In order to do so, I'm hoping to live off of my dividend income from my investments, at around 2.75% or so overall yield (with some higher-yielding fixed income thrown in to bump up the overall yield). This would permit my income to grow somewhat with inflation over time (as companies raise their dividends, when their profits should hopefully grow with inflation).
I'm unfamiliar with tax rates in the UK. One thing to keep in mind - do a sample income tax return for yourself to see what you would owe net in taxes if you lived solely on dividends/interest/capital gains income. In the US, people are surprised to see that all of those taxes they paid while working (Social Security, Medicare, etc.) help make their net average tax rate noticeably lower, so they don't need as much income from dividends/interest to fund their expenses.
It is challenging to find investments yielding a higher return, but they do exist. The first step is realizing that firing a financial advisor is probably the first step you need to take (if applicable), since their fees are usually not worth it. Next is to simply educate yourself more on what different investment options there are to pick from, and what kind of yields/returns they offer (dividends, interest, etc.)