Obviously you will have your own personal choice on how you would prefer to travel but just so you know, the UK law regarding taxi travel is that if the taxi provider does not offer an appropriate car seat, it is still legal for a child to travel in a taxi but they must be in a rear seat and use an adult seatbelt if they're over 3 or no seatbelt if they're under 3. This varies by country as well so always check the laws of the country you're travelling to if you're planning to use a taxi there. X
If you're using taxis hold them on your lap as per the law if the driver doesn't offer a car seat
You need a car seat that is suitable for the baby’s age. If you rely on the taxi service to provide one they are often for toddlers and not safe. You can take a car seat as luggage for free with most airlines if you are flying.