A suit is an investment, Lozey. I would buy the best you can afford. It will last for years. A little investment now and it will pay for itself many times over. I occasionally make my own clothes and did consider trying my hand as a tailor's apprentice when I was younger. It's a bit of an 'area' of mine. I own several suits, so many that I have actually ran out of space for them and cannot buy anymore

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Most of these are ready made, but professionally I would
never go for an off-the-peg suit. Go bespoke if you can. Especially if you are to own only one suit. There simply is no comparison. Not only will a made-to-measure suit fit you perfectly and flatter your figure, but it's a complete one-off. Nothing that a cheap polyester off-the-peggy can do. You can even ask for extras like secret pockets. The prices aren't too bad either, certainly less half the price of a top designer suit. I've had prices around £250. Unfortunately, the off-the-peg suit has taken over the universe.
A made-to-measure suit is a bit like hard drugs. Once you have tried one, I'm afraid there's no going back. Except that you can't pay for drugs on your credit card.
If you want to stick to the high street though, then for £100 you're probably best off trying somewhere like Matalan or some of the factory outlets of large high street stores like Marks&Spencers, although on my last few inspections the standard of the clothes had been dropping.
Dewhirst's do a good suit, if you're lucky enough to have a factory closeby.
For £100, your best bet is probably TK Maxx. They do lots of remainders and samples. I have several one-offs from there that are brilliant quality. You can get a well made, pure wool suit with all the nice bits for under £100.
And don't forget the charity shops, but that can be a lot of hassle.
Without a doubt my favourite way is Ebay. I would avoid this, however.
It's far less hassle and the choice is massive, But the thing is, I do this as a hobby and I can justify taking the usual Ebay risks. If I buy something that is awful 'in the flesh' I just pass it on. Also, there are a few things that are essential. You must know your measurements perfectly. And I'm not talking just chest size here, but sleeve length, shoulders, waist, jacket length etc.
I always buy
slightly larger and get a tailor to adjust everything to fit. This can cost as much as I paid for the suit, but I still end up with a bargain.
My advice? Try and find a remainder store or a factory outlet like TK Max. The stock rotates every so often, so pop in a few times over the weeks.
Find something that fits and that you feel comfortable in.
Don't buy anything too fashionable. Those wide lapels and thick lurid green stripes might feel spot-on now, but you'll feel like a right idiot at your cousin's wedding in four years time.
Stick to plain colours. If you are only going to own one suit, then black is best I'm afraid. You'll be able to wear this during the day or at an evening function.
Buy pure wool if you can, in a medium weight. You can then wear it all year round.
Finally and most importantly, get a good tailor to make alterations or add those irritatingly missing pockets. As Dat says, they'll be quite reasonable.
Mark