Do not park at Heathrow for a night out in London. Don't even think about it in a nightmare. It is VERY expensive (cheapest Long Term car park is over ￡21 for 3 hours) and you will then have a 15 minute shuttle ride into the Central Area on a noisy and rough old bus. Then you have to either suffer the crowded Piccadilly Line into central London (40 minutes) or the expensive Heathrow Express (about ￡24 per person) to Paddington (about 15 minutes) - both routes journey times could be 10 minutes longer if you get to the platform just as a train is leaving and you have to wait for the next one.
Trains into London from most places are fairly good value depending on where you are coming from and whether you are travelling off-peak or not. You should check the internet sites of your local train operating company to see what deals are available. Pay in advance tickets are normally up to 75% less than the turn-up-and-travel prices.
If there is a party of you and you live in the south east then consider buyng a "Network Card", which gives discounts for the card holder and a companion. You may find that the savings over a normal Day Return ticket more than covers the cost of the card - and you can use it over and over for up to a year.
http://www.network-railcard.co.uk/
An alternative to train the whole way is to drive to one of the outer suburban or "satellite town" stations and park there. Pick somewhere reasonably big so you get a better train service (e.g. Slough or Ascot or High Wycombe if you're coming from the West).
A lot of out-of-London railway stations offer free off-peak parking, or only charge a few ￡￡ for a whole day. But make sure you understand when the day's parking expires - mostly it's at midnight - and you will have to pay again for the following day even if you leave the car park a few minutes after midnight. The cameras will catch you so you're not safe from this even if there is no attendant.
The simplest way is to go by train from Hereford to London Paddington either on a direct service or by changing at Newport. The cheapest tickets are those booked well in advance for specifically timed trains, so do so as soon as possible.Not via the Trainline as suggested in another answer but at:-
www.firstgreatwester.co.uk (no booking fees or credit card charges), or at the ticket office at Hereford Station
An Off Peak Return not booked in advance is from ￡55 to ￡68
An annual railcard will give you a further 34% discount whether you book in advance or not www.railcard.co.uk
Parking and indeed driving in the Greater London area is not advisable
The best option by far is to take a train. And the place to look for fares - and book - is the train company's web site https://www.firstgreatwestern.co.uk/ Most likely you will need to change trains at Newport. You arrive at London Paddington.
Fares vary by time of day so I suggest looking it up yourself for train times that suit you. The cheapest are Advance tickets - you will need to book two singles - but by now these will be running out for 20 March as there are only a limited number available. So book now! You have the choice of having the tickets posted to you or collecting them at Hereford station.
Parking in London is just plain expensive, and Heathrow is the worst. You CAN use it without using the airport, but better to let the train take the strain and wherever your friends are, London public transport will undoubtedly get you there from Paddington - there's so much of it. Ask your friends for advice on that.
If you can not go by train the whole way, the cheapest option will be to find a safe place to park that does not require you to pay, or as little as possible.
Near all airports are the cheaper parking option, do read reviews as not all are as reliable as you would like.
The official airport parking lots are more expensive but likely still cheaper than parking in central London, (in addition to that charge they ask of any car coming in.)
I would personally look in a small town on the edge of Greater London, with either a railway or an underground connection. Likely many of them have multistory car parks that have a 24 hour rate that is much cheaper than the airport ones.
Get the train. It's too expensive to park in London and there is a congestion charge on weekdays.
Get the train from Hereford!!! You can find FARES on the trainline.com
