It's definitely worth having one.
You mention it needing to be compatible with your pushchair - I wouldn't be so sure about this. It really depends on how you're likely to be using it. What would be your usual scenario of using a taxi, do you think?
For example - if your taxi/lift scenario would involve you being picked up from home, travelling to a place, staying in that same place for some time and then going home again in (any) car, OR being picked up at home, travelling to a place, not necessarily staying in one place but being picked up again in exactly the same car again later, either of these would allow you to use a more bulky car seat which you could either uninstall and keep with you, or leave installed in the car you're getting a lift back in later (taxi drivers can often keep the seat for you in their boot/at their office if you have booked the same specific car again for later). In this case, there is no need for compatability with your pushchair, meaning you can get the absolute cheapest (safe) seat on the market which is the Joie Juva, Mothercare Ziba, or Nania BeOne SP. These are all around the £35-40 mark. Personally of those I would go for the Joie, but the other seat does fine in independent crash testing.
Another option if this is your situation would be to spend a little bit more and get a combination seat which goes from birth to about 3-4 years such as the Joie Tilt, which again doesn't cost very much money (about £60-70) but will last you longer so is probably better value. The infant carrier seats with handle only last until about 12-18 months. The other bonus of this is that it's one of the cheapest stage 2 seats around which is actually safe, as many of the very cheap seats in this stage are very poor performing in crashes. I would then get a separate lightweight high backed booster after this seat is outgrown because this will be more practical than a seat like the Joie Every Stage which ends up quite heavy and bulky.
On the other hand, if your situation is more that you tend to take public transport or walk part of the way somewhere and then get a lift, or walk/public transport one way and get a lift back (or vice versa) or you tend to get lifts in various different cars over the course of the day and move around between car trips, then it probably would be useful to have the possibility to stack the car seat onto the pushchair to cover both possibilities without having to carry the seat around. In this case, it's useful to know the following:
Infant car seats which are "universal" ie fit on any pushchair which states it takes a MAXI COSI or CYBEX seat:
Joie, Maxi Cosi, Cybex, Nuna, BeSafe, Recaro, Venicci, GB
(Except: Joie Juva, Recaro Zero 1 Elite)
Additional Pushchair brands taking a "maxi cosi" (universal) adapter:
Silver cross, Cosatto, Bugaboo, iCandy, Uppababy, Stokke, Quinny, Mamas & Papas, Mountain Buggy, Phil & Ted's, Mutsy, Britax
Car seats/pushchairs which ONLY fit their own respective prams:
Britax, Graco, Silver cross, My Babiie, Ickle Bubba, Mothercare, Chicco, Hauck, Jané, Tutti Bambini, Kinderkraft
(Except: Some non-Britax prams have a Britax adapter.)