As with most things there are several ways to tackle bike carriers. Here are my thoughts
...
Rear deck bike carrier
The old style racks which sit on the glass and boot you need to be careful with in my opinion. Both from a weight perspective, damage perspective ( damage to car paintwork, damage to spoiler, glass and rear deck ) and obstruction of lights etc. Yes they are cheap, but If you have a rear spoiler you probably cant use one. I have one and would not use on my current car. When I did use it, I always had to check the front tyres werent rubbing on the rear quarter, quickest way to loose some paint!!
Tow bar bike rack
A tow bar solution is the most effective if you want to carry more than 2-4 bikes, and the ones which swing down and out of the way enabling you to gain access to the boot are superb. Downsides are you need a towbar which are not cheap to buy or have professionally fitted and they can cause havoc with parking sensors and rear visibility. They are also relative big units which you need space to store when not on the car.
Roof rack
I know you do not have the roof bars, but you can still buy racks which fit to the door seals. For carrying 2-3 bikes this is probably the cheapeast and simplist solution. Fitting a roof rack takes just a couple of minutes once you have mastered and you keep the full use of your boot and all visibility. On a normal height car ( ie not a 4X4 ) I dont see you having any height issues, unless you really do enter some super low car parks, in which case its a 2min job to drop the bikes of the car. Take a look on Ebay for a used set of decent Thule bars. The plus side of a roof rack is you can also fit a box or use them to carry other items other than just bikes.
Also take a look at
www.roofracks.co.uk , as they sell all of the options listed above, as well as take a good look at Halfords offerings.
I personally have the roof bars and make full use of them with the Thule bike carriers, which are super easy to fit and do everything that I ask of them. I can put the bars and bikes on the car now all within about 5-7 mins on my own. The new version carriers can also cope with mountain bikes, and oversized tyres and are easy to reverse to carry bikes opposite ends.
Heres a pick ( not mountain but road bikes ) making full use of the ‘Sportback ‘.
View attachment 148005
View attachment 148006