Used road bike recommendations: £500

Some good advice already, I would factor in some leeway in your budget for maintenance.

To give some context, the cost for me to replace some items on my old bike this year including bottom bracket, new chain, brake cables, inner chain ring, tyres and one brake calliper this year, approx £270. On what I consider a well maintained bike, that is probably only worth £500.

I agree with the other posts about checking wheels and drivetrain - safety critical items.  And especially with the shortage of drivetrain components, worth making sure it all works properly.  Also, don't forget that fit is important!

During lockdown I accidentally bought a Cannondale Synapse and have put 2000km on it.  A set of Vision 30 wheels made it feel like a new bike for £130.

This seems worth a look though - perhaps too cheap almost, check if they have the original receipt... https://www.ebay.co.uk/itm/144552178540

Big issues on second hand bikes: worn out wheels - with rim brakes, wheels are wearing parts, so depending on how they have been ridden, a bike that is a couple of years old could need new wheels. Some wheels have wear markers, but avoid any bike with deeply dished braking surfaces. Wobbly wheels are easily repaired by changing bearings, but although bearings are not expensive, the labour might be.

Worn out drive trains. Chains are wearing parts so if not changed regularly (depends a lot on type but road bikes can be 1000-3000 miles for a chain and 3 chains to a cassette as a rough guide) you can be looking at not just a new cassette but new chain rings and it adds big money to a bargain bike. 

Headsets and bottom brackets, bearings themselves not usually too expensive, but rarely a home repair because you need tools of the right sort for the bottom bracket and the hassle of working out how to replace bearings and piece it back together for the headset.

I point this out because in the second hand world there are rarely bargains for the buyer who doesn't know what to check to find out whether what they are buying matches the description - a bike mechanic can spot the big problems quite quickly, even though a worn out bike can look remarkably like a good find.

I'm a fan of Giants, and they do not generally attract the brand premium of Trek and Specialized even though they are of similar quailty. The alloy Defys can be comfortable and good value.

Aside from that there are a miriad of decent bike brands and it is very difficult to suggest what you might hit on. I think the secondhand market is cooling down a bit from the difficult days of a couple of years ago, so my suggestion is be less worried about brand and more concerned about a good fit. 

I'd say stick with Btwin if you're going secondhand, 500, 520 or 540 all excellent bikes and can be had on eBay for prices that leave you enough change from £500 for a wheel upgrade if you fancy it. 

An Allez is probably going to cost more than an equivalent Btwin due to brand snobbery.

My rule of thumb on used bikes is that the make and model is a lot less important than the condition. A tired drive train could easily cost you another £100-150 to sort out. So a nicely looked after aluminium bike with no suspiciously crazed paint (which would indicate crash damage), a clean chain, gear changes that just snick satisfyingly and reasonably true wheels but a humble groupset will probably do you better than a filthy wreck with fancier spec. And probably less likely to have been nicked - inspect seller as well as bike.