comparing to stuff like cycling, photography, power tools etc where its quite easy to see the difference between £100, £500, £1000 and £2000+ products
Different features are not only associated with cost but also with the "series" of machines - you've already noticed the difference between the FS and F series machines.
Within a series of machines, an extra £50-100 can give you additional stitches and perhaps features such as a hard case, needle threader, automatic thread cutter, etc. The differences between series may be features such as fonts and/or different alphabets, a larger machine with more space to sew bulky items, adjustable foot pressure for different types of fabric, the option to mirror stitches, a more powerful motor, knee lift etc.
Have a look at the Brother machines on the
Sewing Machines Direct website as they have a useful "compare" function.
If you select a £100 machine (for example, L14s or LX17), a £500 machine (for example, A80 or A150) and a £750+ machine (for example, NV1100) you will be able to see the different features associated with each series of machines.
If you've tried editing photos or working with a large document or spreadsheet on different computers, you will know that the task is made easier with a bigger screen and more computer memory and that some computers will struggle with the task or may crash. It is a bit like that with sewing machines - a basic machine will sew fabric (but may struggle with thick fabric) and extras will make the task that bit easier and more enjoyable.