It depends on your perspectives on risk and what your priorities are. I've no doubt that if everything goes perfectly you will make the connection without a problem.

Arriving at LHR that early I would personally prefer the opportunity to grab a shower, change clothes and something to eat before heading off to my final destination. You will feel altogether more human for it, especially as ARN is over two hours away. So, if you don't particularly need to be back at ARN early then I would be minded to build in a two hour stopover, which is ideal to shower, change and eat without feeling like you've wasted time.