If “FIRE” is what you have in mind, ideally you need to be doing both, attempting to save 50% of your earnings or more, including your pension. Use a Stocks & Shares ISA and a general investment account if £20K per year isn’t enough, to fund the first part. The pension covers the second part.
Mr Money Mustache has some great advice on this, although you need to translate it over from American. They don’t have ISA’s, so their investments are taxable; their IRA’s and 401K plans are like our SIPPs.
The Shockingly Simple Math Behind Early Retirement (mrmoneymustache.com)