This is most definitely bookable - it's what's called an "open jaw" ticket, and will normally be priced the same as a return ticket. What's more, you could probably book your return flight back to your actual destination for roughly the same price, although this can vary slightly based on the actual airlines and the specific cities.
You'll find more details on these types of tickets in the answers to the following questions :
What exactly is a open-jaw flight?
What are the "rules" for multi-city flights?
One-way versus return airfare tickets
As far as what will happen if you do fail to take your last segment, you'll generally be fine - unless you've checked luggage. Any luggage you check will be checked all the way through to your final destination (AMS). When you fail to board the LHR-AMS flight, your bags will have to be removed from the plane, and you will have to spend some serious time in order to reclaim them from the airline.
There can also be issues in the case of irregular operations (ie, delays, breakdowns, etc) in which case your airline might re-route you via somewhere other than LHR in order to get to your final destination of AMS.
Again you can find more information in questions such as Do you have to take the second leg of a flight?