The keys used on the Omega and many other cars operate in a "License Exempt" frequency allocation at around 433 MHz.
This part of the spectrum has many other such devices operating in it. Radio controlled toys, garage door openers, cordless weather stations, cordless mice and keyboards, etc..
Furthermore, it has some bigger boys operating there. UK radio amateurs are allowed to transmit up to 400 watts on the 70cm band (430-440 MHz) and military usage is also present.
TETRA base stations used by commercial and emergency services have an allocation close to this frequency and may well be transmitting tens or hundreds of watts.
An Omega keyfob is limited to a maximum of 0.01 Watts so you can see that this signal is easily swamped by transmissions from some sites. Often base stations are located on the tops of multi-storey car parks and railway stations where they give good coverage of an urban area so it's entirely expected that you might come across areas where the fobs don't perform well.
It's well worthwhile checking your manual lock barrel still works every so often.

Kevin