Well you could go by coach. I just googled 'coach holidays to Benidorm' and there were quite a few, for example http://www.door2tour.com/benidorm-coach-holidays.aspx
Although I'm sure it'd take almost a whole day to get there! I've done coach trips to Paris and they can be quite uncomfortable so I don't know how your Father would cope!
I suppose there are ways of taking the train to Paris via Eurostar and then beyond into Spain etc but it's not something I've done!

there are many coach firms that include benidorm in their schedules,
http://www.ferriscoachholidays.co.uk/
being one.
they do ferries to bilbao then coach through spain.
http://www.siestaholidays.co.uk/
also go there.
there is plenty more information on these companies on the costa brava forum, and info on coach travel to spain in general.

Some time ago there was a thread on HT which gave links for travel by train from UK to Benidorm. From memory you could also book sleepers.

fwh

You can drive with an overnight stop. However getting a place on the Tunnel or a ferry could be a problem.

The website with info about trains to Spain is http://www.seat61.com/Spain.htm. We quite fancy trying it some time.

That is the site shown in the previous discussion. I passed it to a friend of mine who has family there. She has decided that flying is no longer what she wants to do and is considering using it if she decides to go again. She is in her 80s.

fwh

There's a lot of things that could go wrong on a journey like this with someone of that age.

let's just highlight some details of the rail journey, the brain has a habit of blocking out the bits you don't want to see!

First, you have to get to London St Pancras - that's often the hardest part for many people!

The Eurostar train arrives at one station in Paris and the onward connection goes from another, you have to sort out the transfer (I think taxi would be best).

If you go the TrenHotel route via Barcelona the same applies there although the transfer should be included by local train.

If you use the Lunea service you avoid Barcelona but have to change at the border.

In Alicante the local service to Benidorm is a tram and goes from a different station, the walk is about 500metres and the tram stop is underground. The tram ride is over an hour and there is no toilet.

I would of thought going by coach rather than plane would be far worse for your dad.
It takes at least 24 hrs (depending where you are in the UK). You often need to get off the coach and transfer to another before boading the ferry at Dover, off and on again on the ferry and again at some point in Spain, and need to move all your luggage at these changeovers.
Whilst these coach trips are cheaper and mostly adventurous, which we have done several times in the past, we wouldn't contemplate it with an elderly family member.

I've crossed Australia by coach many years ago without any problems. travelling with both Australia Pacific and Greyhound, but when I went to Italy by coach some years later I didn't find it particularly comfortable. My leg swelled up both on the outward and return journey and although it soon went down when I got there, coming back the swelling didn't go down for over a week.

every time i travel to spain by coach i wear the support stockings that are used in hospitals when you have an operation.
you can buy similar in shops (boots sell them) and they are ideal for minimising your chance of getting Deep Vein Thrombosis.
i would not travel on the coach without them.