With a 100m feedline, your loss target is not too hard to meet — RG-8 has a loss of about 2.5dB per 100m at 30MHz (lower at lower frequencies).
You'll have to compromise on the cost target, though. I don't know much about UK prices, but here in the US, RG-8 currently goes for about \$0.90/foot in bulk, so right around \$300 for 100m of it, not including connectors. Shooting for under \$100 puts you in the range of RG-58, which has a loss of around 8.5dB/100m at 30MHz, and 6dB/100m at 15MHz, which is less fun. $50 won't even get you that.