Sorry for the long post!
I'm not sure what your target retirement figure is, but a staged approach might be worth considering taking full advantage of the tax allowances? Adding these up can give a reasonable tax free income in retirement and may help you increase your savings rate before you FIRE...
The Personal Allowance is £11,850, the Interest Allowance is £1,000 (if lower tax rate, but tapered for those earning more) and Dividend Allowance is £2,000 plus you have an additional Saving Allowance of £5,000 if your income is from savings alone (in FIRE) and less than £18,850?
You also have the annual £11,700 Capital Gains Allowance, £1,000 Entrepreneurs Allowance (e.g Ebay selling), £7,500 Rent a Room Allowance (per house only) and £1,000 for Other Property (e.g rent your driveway, tool share etc.). If you are married and one of you pays the basic rate tax and the other is a non tax payer, they can donate £1,185 of their Personal Allowance to the basic tax payer.
You also need to make sure you pay at least 10 years of National Insurance to get any State Pension so keep an eye on this as you can back pay Class 3 NI if needed and you'll need 35 years to get the full payment. Having this provides your 'floor' i.e. a set income you can rely on in your old age.
You should obviously aim to get the maximum pension match from your employer and some employers will allow you to salary sacrifice to your pension, which could be useful if you are a higher tax payer i.e. bring you down to a lower tax band. I've also heard that some employer's pension schemes will allow you to partially transfer to a SIPP annually or you could do that when changing jobs as I assume you don't have a DB pension and this might reduce your annual fees.
Using your ISA allowances each year is important (£20,000 this year), but be careful about fine print on LISAs as you may get hit by a penalty if withdrawing early (larger than the tax advantage you obtained). You can have a S&S ISA and IFISA in the same year as long as you stay below the allowance, but can't have a cash ISA too.
If you have excess money available for investment, then the next priority would be non tax shielded accounts for interest, dividend income and capital gains. You need to keep good records for these to fill in your tax return. Monevator has some good articles on this e.g. INC not ACC, Bonds and Shares are complicated outside an ISA or SIPP etc.. If you are a lower tax payer, dividends over the allowance are taxed at 7.5% rather than the 20% tax on interest over the allowance. Watch out for changes in how cryptocurrency gains should be declared and it may be worth draining each year to keep within your CGT allowance. Bullion gains would also count for CGT...
Extra income could be gained from taking in a lodger, ebaying etc and used to raise your investment rate if you expect to beat your mortgage interest rate. It should be more tax efficient to have your mortgage paid off before you finish employment, but keep an regular eye on this as you remortgage as LTV can impact your best mortgage interest rate. You may have problems remortgaging if not earning, but some people may choose to take a longer fixed rate mortgage at a worse interest rate just before retiring early to give some flexibility e.g. using pension lump sum later to pay off the mortgage rather than using non pension funds.
At FIRE you may choose to have a combination of cash, bullion, and cryptocurrency; investments ouside an ISA or SIPP; various flavours of ISA; SIPP, employer's pension and State Pension. You need to have enough to draw from non pension income until your pensions kick in.
Be careful about taking your Pension Lump Sum early if you plan to work at any point after this, as doing so limits how much pension you can pay in to just £4,000 per year. If you are not working, you can pay in £2,880 to a SIPP each year (made up to £3,600 through tax relief) until age 75, which will help the 'stache?
Hope that helps?