CB, DIY and the law.

Rules.
Citizens Band radio in the UK has very few rules. However, there are rules regarding equipment used and specifications. Rules can change from time to time, as they have done over the last few years since CB was legalised in the UK.
This page is not intended to be a full explanation of the law regarding CB, but is just a guideline. Further details of CB rules may be found on the Ofcom Website.

The radio.
Radio transceivers used for CB radio must be to the legal standards set by law - and must be tested and approved before they can be put on sale. This testing and approval procedure, along with the costs, ends the chances of anyone building their own equipment.
There also some restrictions regarding what can be combined with a CB radio in a single unit.

Although anyone is allowed to repair a CB radio, there are restrictions on what you are allowed to do. You may not add extra channels or bands, increase the power output - or make any other modification that would make the radio work beyond the allowed specifications.
Although dual band radios are available, you may not add the extra channels to a radio designed to have just the one band.

Tuning up the radio.
You may tune up a CB radio, as long as you do not take it beyond the legal limits. The main points would be power output and deviation. As far as the receiver goes, the main specification would be adjacent channel rejection. Recent sets have a minimum rejection figure set by law. As this was put in place to protect you, the consumer, from cheap rubbish radios, you really do not want to upset this - so crystal filters should be replaced only with the same type.

Radio add-on boards.
Any modification or addition to a CB radio which will impact upon the specifications tested will make that radio illegal for use in the UK. This includes any adjustments or add-on boards designed to alter the operating frequencies or power output.
As pre-amps are illegal for CB use, adding a built in RX amp would also be illegal. Any other add-on which would take a CB radio beyond it's allowed specification would also be illegal. A radio that has been designed and accepted as single band cannot be converted to dual band.

Microphone circuits.
As the maximum allowed deviation is set by law, and most radios will already be as close as possible to this, any microphone amps could easily take the radio outside of the legal spec.
It should be noted that, with just 10KHz between channels, any boosting of your outgoing audio can soon start interfering with other CB channels -which will not make you too popular.
If using some add-on microphone circuit, care should be taken to ensure that the deviation is set correctly.

Antennas
Although there are rules regarding CB antennas, they do not need approval before being sold or used. This means that there is no legal reason why you cannot design, build, and use your own antenna.
The only requirement is that it conforms to the simple rules set regarding length, width, and directivity.