If you're only confused by the word 'datum', that's what one of those is - quite a widely used term, I thought. Typically, the main lines radiating from London would measure mileage from zero at the London terminus, increasing away from London.
For reference, here is a hopefully complete list of lines with increasing mileages from Paddington:
Great Western Main Line (Paddington to Penzance via Box)
*Acton and Northolt Line(Old Oak Common West to Northolt Junction)
*West Ealding Loop(West Ealing Junction to Greenford West Junction and Greenford Bay)
**Greenford East Curve(Greenford South Junction to Greenford East Junction)
*Heathrow Link Line(Airpport Junction to Heathrow Terminal 4 and Terminal 5)
*Staines Branch(West Drayton Junction to Colnbrook)
*Windsor Branch(Slough to Windsor & Eaton Central)
*Wycombe Branch(Maidenhead to Bourne End)
*Henley-On-Thames Branch(Twyford to Henley-On-Thames)
*Basingstoke Branch(Westbury Line Junction to Basingstoke)
**Berks and Hants Line(Southcote Junction to site of former Stert Junction)
***Stert and Westbury Line(site of formet Stert Junction to Westbury North Junction)
****Westbury Avoiding Line(Heywood Road Junction to Fairwood Junction)
****Westbury East Loop(Westbury East Loop Junction to Hawkeridge Junction)
****Castle Cary and Langport Line(Castle Cary Junction to Cogload Junction)
*Didcot East Curve(Didcot East Junction to Didcot North Junction)
*Didcot and Chester Line(Chester Line Junction to site of former Handsworth Junction)
**Oxford, Worcester and Wolverhampton Line(Wolvercot Junction to Round Oak)
***Worcester and Hereford Line(Shrub Hill Junction to Shelwick Junction)
***Madley Junction to Ironbridge(Ironbridge to Lightmoor Junction)
***Stoke Branch(Droitwich Spa Junction to Stoke Works Junction)
**Leamington Spa and Coventry Line(Leamington Spa Junction to Milverton)
**Bordersly to Tysley Line(Small Heath South Junction to Bordersly Junction)
**Handsworth to Stourbridge Junction Line(site of former Handsworth Junction to Smethwick Junction)
***Galton Junction to Stourbridge Junction(Smethwick Junction to Stourbridge North Junction)
**Wolverhampton to Saltney Junction(Wolverhampton North Junction to Saltney Junction)
***Madley Junction to Ironbridge Line(Madley Junction to Lightmoor Junction)
*South Wales Main Line(Swindon to Johnston via Gloucester)
**Llanwern Works(Llanwen Works East Connection to Llanwern Works West Connection)
**Western Valleys Line(Gaer Junction to Park Junction)
**Newport Low Level Loop(Alexandra Dock Junction to Newport Docks)
**Roath Dock Branch(Pengam Junction to 168 M 61 Ch)
**Swansea Disstrict Line(Briton Ferry Up Flying Loop Junction/Court Sarl Junciton to 208 M 49 Ch)
**Swansea High Street Branch(Landore Junction to Swansea)
**Carmathan and Newcastle Emlyn Line(Carmathan Junction to Carmathan)
**Pembroke and Temby Branch(Whitland Junction to Pembroke Dock)
**Claberston Road and Letterson Line(Claberston Junction to Letterson)
***North Pembroke and Fishguard Line(Letterson to NR Boundary)
****Fishguard Harbour Lines(NR Boundary to Fishguard Harbou)
**Milford Haven Branch(Johnston to Milfrod Haven)
*South Wales and Bristol Direct Line(Wooton Bassett Junction to Patchway Junction No. 2)
**Filton East Curve(Stoke Gifford Junction to Filton)
*Weymouth Line(Thingley Junction to Dorchester Junction via Frome)
**Salisbury Branch(Westbury South Junction to Wilton Junction)
**Frome Avoiding Line(Clink Road Junction to Blatchbridge Junction)
*Bristol Loop(Feader Bridge Junction to Dr Days Junction)
*Portishead Branch(Parson Street Junction to Portbury Dock Junction)
**Portbury Dock Branch(Portbury Dock Junction to Portbury)
*Weston-Super-Mare Loop(Worle Junction to Uphill Junction)
*Torquay Branch(Newton Abbot West Junction to Paignton)
*Sutton Harbour Branch(Laira Junction to former Friary Junction)
**Friary Branch(Former Friary Junction to Plymouth Friary)
*Lostwithiel and Fowey Branch(Lostwithiel Junction to Fowey Dock)
*Par Loop(Par Loop Junction to St Blazey Junction)
**Newquay Branch(St Blazey Junction to Newquay)
*St Dennis Branch(Burngullow Junction to Parkandillack)
*Falmouth Branch(Penwithers Junction to Falmouth Docks)
*St Ives Branch(St Erth Junction to St Ives)