Your first port of call should be the SF Primer 101
What do you understand by "tailor made"? MTM? Bespoke?
MTM uses an existing block (pattern) and alters it based upon your measurements. You choose stylistic features, material, etc.
Bespoke uses a unique block based upon your measurements. You choose stylistic features, material, etc.
In between the spaces lies your body shape, budget, experience, plus the important relationship with the tailor in terms of understanding and interpreting your requirements. Like most things in life, the bigger the budget the wider range of options available (not necessarily a better result).
That said, you can forget about sub-£500 for anything remotely decent in the MTM world. Double your budget and you'll have more options and might, just might squeak into entry-level bespoke (although you'll probably have to double again).
Unless you greatly desire a tailor made suit and/or have a body shape that requires either MTM or Bespoke, then stick to RTW and an alterations tailor.
Suit Supply is the obvious choice at your budget. You also have Charles Tyrwhitt and TM Lewin. You can also try M&S or Ede & Ravenscroft (on sale £400 for half canvass with extra pair of trousers). All are RTW at your price point.
You may wish to try Dress2Kill or A Suit That Fits - no experience with either company, but I do roll my eyes at their claims of bespoke starting from £325 in the Metro.
Read the RTW Brand Review
thread. You'll get a better idea of what you should expect for your budget.
Note: fit and proportion (your body shape should determine the size of your suit elements i.e. lapel size, gorge, button stance, shoulder type, etc) are the most important considerations.