The U.K. has made an allocation just above 900 MHz as well as 27.6 to 29.9 MHz. Typical values of maximum transmitter power allowed is 5 watts of FM and AM and 12 watts of SSB, though some countries limit the power to 0.1 watt. The result is that legal CB is meant to be for local use, typically up to 8 kms range. But if one or both stations are on high ground then longer ranges are possible. Also if the transmitter power is increased to 40 or 100 watts or if beam aerials are used ranges up to 30 kms. may be achieved.
Some sets are capable of producing all three types of modulation and have a selector switch.
A FM transmission cannot be understood on a AM receiver or a SSB transmission on a FM receiver. Therefore you should ensure that any CB radios you obtain all have the same modulation system. For a few years, each time the sunspot number is around its maximum, CB can be used for long distance contacts between countries and continents. Such contacts are often not within the terms of the CB licence. CB radios are readily available and often at low cost so depending on your distance requirements CB may be an economic solution.