You should be ok - you're not actually entering the UK.

i agree with siasl but it may depend on where you are comming from as if that would be allowed i would say contact customs and excise and see what they say

It depends if you are flying from somewhere within the EU. Under new rules, any persons travelling from OUTSIDE the EU are not allowed DIRECT entry into an EU airport. You will still need to pass through border controls and cannot go directly to the transfer area.

I have found this a pain when travelling from Egypt, Turkey and Russia through Germany to the UK. We need to pass through passport control in Germany ... no problem as I have a UK passport but the German's like their short transit times!

London has several airports so you should make sure that the Ryanair flight to Sweden leaves from the same airport as your other flight arrives at.

See also:

http://www.ukvisas.gov.uk/en/doineedvisa/visadatvnationals