Jumping Jack - to legitimately avoid CGT, register the property that you are letting (and the one that you might sell first) as your 'Principal Private Residence' therefore there shouldn't be any CGT to pay of you sell it. You can then flip the PPR to the next property - jsut like MPs do/did. Keep the letter in a safe - it might be worth 10,000s GBP - HMRC came after us - and had lost the original PPR approval. I served a notarised copy on them (and as mentioned above, got a buttock-clenching appology form them).
Cleared to roll:
Losses cannot be offset against your other (salaried) income. Up until 1 Apr 11, you could do this for Holiday Lets, but EU ruled that this was illegal. HMG did fight this one - but lost. Losses can be rolled forward to offset gains in future years.