Recommendations on Car Hire and Train Travel for a UK trip in March 2013?

The best places to hire a car - with the widest choice of cars, most deals, most companies etc., will be at airports, particularly Heathrow. I'm not clear on what you mean by 'catch a train out of London and hire a car', but if you stop off at Heathrow, which is not far from central London, you can pick up a car there. There are a variety of ways to get there, although a taxi may actually be the cheapest and easiest if you have your entire family and a lot of bags.

You can also pick up cars in the centre of London (edit: don't forget the congestion charge) - most of the major rental companies have several locations - but check opening times etc. Wherever you rent from, I'd advise making an advance reservation to avoid disappointment.

If you meant to take a train out of London, then pick up a car, your options will be much more limited, unless you travel to another major city. Car hire locations are typically near airports and major train stations. The train system in Britain is generally OK, although you may find it a bit expensive. Travelling to rural attractions without a car will be almost impossible however, in many cases, unless you are prepared to spend a fortune on taxis.

I don't see any major practical problems with your plan. If you are used to LHS driving, you should be fine. You can rent an automatic if you need it (make sure you make that clear with your reservation as manuals are normally the 'default' in the UK), although manuals will give you more choice.

If you are driving around small towns, remote areas, etc., it's worth getting a guide to Bed and Breakfasts (aka guesthouses, typically in a private house) in the UK. In March I would expect many won't need advance booking, but it's definitely still worth calling ahead on the day you might plan to stay.

Edit: one more thing - although signage in the UK is generally excellent, roads are narrow and twisty in many places, so you'd be advised to get a GPS unless you are confident that you are travelling with a strong navigator and a good map. Saves on arguments in my experience :) Most major rental companies will rent you one, of course, although it might be cheaper to make your own arrangements.

Enjoy your trip!

First off 2.5 weeks is a little short but certainly possible though you may find yourself doing a lot of driving and not a lot of seeing, I'm assuming only Northern Ireland.

Have a think about what you want to see in England/Scotland/Wales/Northern Ireland

If you are heading to each of those places just so experience the culture difference then short trips with a major city plus some interesting small towns and sights will do.

If there are specific things you want to see this could strain your budget in terms of time for example playing a round of golf at St Andrews and going to the isle of Skye would suck up 4 or 5 days.

British A roads(motor ways) are very obscured from scenery so you wont see much while driving unless on a B road (narrow country lanes (often 1.5 or 1 lanes wide)

If you are going to train out the city to pick up a rental car you have two options:

1. You can train to a commenter town that has a branch of your preferred rental company and go from there Luton or Milton Keynes would be two such towns. This disadvantage of this is getting from the station to the rental car office could be a long work or taxi.

2. Train to the first major city and and pick the car up there. Both Hertz and Avis will do railway station drop off's in major city's such as Edinburgh.

For intercity train prices National Rail is the best starting point. If you are doing a long inter city train such as London to Edinburgh go to the website of the operating company (in this case east coast trains) and book directly selecting seats. Otherwise all train tickets in Britain are unreserved seating and standing for 4 hours on a packed train is uncomfortable at best. The price will be the same as national rail, just the option to select seats doesn't always work on that site.

Edit

Weather In March you will likely have snow in the Scottish highlands (I did in May this year) Also be prepared for wind warning on bridges in Scotland. The bridge over the Firth of Forth may close with fairly short notice.

Rail if your children are over 16 you can get a group ticket requires a minimum of 4 and for all people to be travelling together at all times. I'm not sure if you can get them from ticket machines so allow a little extra time and go to the ticket window and ask. I find they normally work out at the price of 2 adult tickets when travelling with 4 adults.