No, this is basically the issue with all of them.
You have three different metrics and you end up having to pick two:
Affordable
Good safety
Portable
So you can basically get one which is affordable and safe, but is bulky and/or heavy and a pain to carry around. Joie Tilt, Steadi, Stages, Graco Extend, Enhance all fall into this category, and there are R129 versions of some of these models, which is an advantage as this is a newer safety standard.
Or you can get one good safety features that are genuinely portable, but a lot of them are heavy, like the RyRy Scallop, or TinySeats, and the main issue with these is they cost around £300-400. There is also Nachfolger Hy5 in this category which is an inflatable rear facing seat, very good safety, very light and portable, but it's expensive.
Or you can get some which are portable and are on the lower end price wise but tend to be non-ideal in safety. For example, Maxi Cosi Nomad - the problem that reviewer is having is called forward anchorage and unfortunately it looks like it would get that in most cars. Urban Kanga is a very similar model and is the same. It also got a very poor Which? crash test. Or you have Chicco Fold and Go, which is suitable from 100cm tall, but most 20 month olds won't be anywhere close to this yet and really, this type of car seat is designed for much older children (around 4 years+) as it uses the seatbelt rather than a safety harness, so doesn't offer good protection for younger children.
There is supposed to be a newer version of TinySeats coming out next year which will be up to 23kg weight limit, which sounds great, but probably a similar weight to the RyRy. And sounds like it will be expensive too.
Unless your 20mo is very tall, you may be best off looking for the largest infant carrier which will fit onto your buggy, Cybex ones are generally very tall in the shell and are compatible with a lot, and then continue to use this for as long as possible. They can safely use an infant carrier if their head is still fully contained within the seat shell (and they are under any weight/height limits). But this is unlikely to fit for long unless your child is very small for their age. Maybe until around 2 / 2.5 at a push.
Another solution is to look at the very basic very cheap, generic type seats that they sell in supermarkets, argos etc around the £40 mark. Because these are so cheaply produced they tend to be extremely light, and you can usually get them to cover the 9-18kg weight class which most 20mo would be in. However, be under no illusions of quality: the issue with these is that they have totally minimal safety features. They are basically no more than a flimsy plastic shell with none of the features of more premium brands. They've passed the legal crash test in order to be sold but some models have been known to fail when crash tested at 40mph instead (the Which? testing criteria). The instructions are frequently confusing meaning that they are often fitted incorrectly, which can be disastrous. They usually don't have a very long rear facing stage (if they have one at all) or any side impact protection. They can be better than nothing, if you're very familiar and confident with how to use them, but it's still a very basic level of protection, and they aren't portable in terms of being able to stow under a buggy, they would have to somehow be carried.
Coaches often don't have enough space and/or the right kind of seatbelt to fit a car seat safely anyway, so the purpose of a car seat on a coach is a sort of "better than nothing gamble". If you are intending to fit a car seat on a coach it's a good idea to check that the coach has 3 point seatbelts and ideally see if it's possible to book the very front seats to allow space.
If you're in London or another city that has black cab style taxis, one option is to put the toddler in their pushchair and secure it in the wheelchair space, facing backwards.
Or it may be possible to locate a local taxi firm who will provide child restraints, and exclusively use this firm and request a seat for the specific journey. However this has the disadvantage that you don't know how old the car seat is or how it has been cared for, and it might well be one of the very cheap/flimsy/generic models.
I think the only thing that the US has that the UK doesn't have is Ride Safer Travel Vest which TBH I am quite dubious about as a solution. You could try to import one and use it anyway, but be aware it is not considered a legal child restraint under UK law. For vehicles like taxis and coaches where you aren't required to use one anyway, this would not legally matter, so it would be up to your own discretion.