Decent road bike for newbies

[Superwomann]

I’ve got an old Halfords one which isn’t great.
Can someone with knowledge point me in the right direction please. I’m looking for a road bike which is half decent, I’m happy to buy a secondhand one and spend maybe around £500, can I get a decent bike for that? I’m looking to do a few triathlons for fun but I stil want to do well, and then just riding once or twice a week for cardio during the offseason.

Decathlon do a range, the cheapest is about 500. We have one and it's pretty good and has lasted well.

If you can avoid getting into titanium gear changers and the like you'll be a lot richer and have 95% of a bike costing 5,000+

Based on my experience you'll lose the 1kg difference in weight between your decathlon bike and your second mortgage bike quickly enough. :-)

Personally I wouldn't buy a new bike at that price point but you should be able to get something half decent second hand for £500. Obviously you're not going to get a super light carbon racing bike with electronic shifting and deep section wheels but you should be able to get a reliable road bike with a mid range group set somewhere around that price that you could stick a pair of clip on tri bars on for your yriathalons and ride as an everyday bike too.
If I had £500 to spend I'd probably look for something from a mainstream manufacturer like Specialized or Giant (Liv is their women's brand) with a Shimano Tiagra or preferably 105 groupset, ideally hydraulic disc brakes, and an aluminium frame with carbon forks. You might find someone selling a full carbon framed bike for that amount but I'd be wary unless it's someone you know and trust. Good aluminium tops cheap carbon and damaged carbon is expensive to repair at best and lethal at worst.
Second hand is always a bit of a lottery, you might hit the jackpot right away or you might need to be patient but I have had some great second hand buys both for myself and my DS. My Winter road bike is a Giant ( Liv ) Avail that I bought on Ebay nearly 10 years ago now for £300. It's absolutely bomb proof and a remarkably nice bike to ride for something that cost so little. It's unlikely to win any races (nor am I!) but it's a fantastic every day bike.

Thank you so much for the detailed information! I shall start looking

What is your height & frame size, OP? Can you give us first half of your postcode? PC in case of collection only.
Summer is a great time to find bargains on ebay and happy to look there to help you choose.

There’s some good second-hand bargains around if keep an eye on Marketplace and the like as people brought in lockdown and not used/upgrading to disk brakes etc. Rim brakes work very well though and easier to maintain, so I wouldn’t discount as can get some good bikes a few years old with them for less money.