I think the best advice on here came earlier on, which is to educate yourself about financial markets and investing. You probably will not be able to purchase the best advice for £100k. You might get sound, generic advice but that is not perfectly adapted to your circumstances.
First, you need to decide how long you are willing to have your money locked up. If you might need it within the next five years then that strongly curtails your options.
Then, one really important distinction to learn is the difference between what you are buying and how it is packaged -- what sort of "tax wrapper" or "type of account"
For "type of account" or "tax wrapper", for most people in UK, there are three main options: taxable account, ISA or pension. With a pension (for example a SIPP), the money is locked up until retirement age, but for most people with taxable income that option is likely to give the greatest tax savings because you can, in effect, invest with pre-tax income. With an ISA, you invest with post-tax income, usually pay somewhat higher fees, but any earnings or gains on the investments are tax free, and you normally keep the ability to take the money out if you need it. With a regular taxable account, you usually get the widest choice of investments with the most competitive fees, but income and gains are usually subject to tax unless you are under certain "tax free" limits in the tax law.
Your decision about "type of account" is separate from your decision around what to actually buy within the chosen account type. Broadly speaking, at the moment, in GBP you cannot get more than 1-2% return without taking some type and level of risk, whether that is the risk of having money locked up and then needing it, or the risk of losing some of it. If you are able to lock up your money for several years, you may be able to get 2-3-4%. To get more than that, you will have to take investment risk, which means the possibility of losing some of money, at least temporarily and possibly permanently if you choose to sell or need to sell after a decline.
The big problem right now is that with inflation being higher than "risk-free" interest rates, there really is no risk-free way to avoid erosion of purchasing power. So your choice is between a slow erosion, or gaining the possibility of keeping up or beating inflation but paying for that by taking the risk of a faster erosion in value.
An IFA, for a fee, can help you to understand the above in more detail, and may be able to help you to feel reassured and to get comfortable with certain specific options. A good one can keep you from buying something that is absolutely terrible; some funds charge fees that are too high, or are too concentrated or too leveraged, and should best be avoided. However, no IFA can change the basic facts as outlined above, and if anyone promises you a 5% risk-free return in GBP then they are most likely lying.
Good luck!